This study investigated the effect of journal writing on Grade 11 post- basic education students' EFL writing skills and writing self-efficacy in the academic year 2015/2016. This is a quasi-experimental study using 2 experimental groups and 1 control group to test the effect of in- class and out- of- class journal writing on students' writing performance and writing self-efficacy. There are two independent variables which are in-class and out-of-class journal writing. The dependent variables in the study are writing performance (namely accuracy, fluency and vocabulary) and writing self efficacy. The sample consisted of three randomly selected intact classes in a post- basic education school in Muscat. There were 87 students who participated in the study. Each of the three classes consisted of 29 students. There were two experimental groups and one control group. The first experimental group did journal writing in -class and the students were given a time limit of ten minutes to finish each journal entry. The other experimental group wrote their journal entries out-of-class without setting time and place constraints for them. As for the control group they were taught writing by the conventional way without receiving the treatment of journal writing. The study aimed at answering the following questions: 1. Does journal writing improve Omani EFL students' overall writing performance? here any significant difference in the writing performance of the students who write their journals in class and those who write them out-of-class? 3. To what extent does journal writing enhance Omani EFL students' writing fluency? 4. To what extent does journal writing enhance Omani EFL students' writing accuracy? 5. To what extent does journal writing enhance Omani EFL students' use of a variety of vocabulary in their writing? 6. Is there any significant difference in the writing self- efficacy between the students who wrote their journals out of class and those who wrote them in class? 7. Is there any significant difference in writing self-efficacy of the experimental groups before and after using journal writing? Participants in the three groups were pre- and post-tested using a writing test. The experimental groups responded to writing self-efficacy questionnaire before and after the intervention. The statistical analysis of the data revealed the following results: 1. The results of the post writing test showed that journal writing- whether in-class or out-of class- improved students' overall writing performance. A comparison between the post- test scores of the in-class and the out-of-class groups showed that there was a significant difference in the overall writing performance in favor of the in-class group. 3. A comparison between the post- test scores of the experimental groups and the control group showed that there was a significant difference in the writing fluency in favor of the in-class group indicating that in- class journal writing promotes students' writing fluency, A comparison between the post-test scores of the experimental groups and the control group showed that there was a significant difference in the writing accuracy in favor of the experimental groups indicating that journal writing whether in-class or out-of class- promotes students' writing accuracy. 5. A comparison between the post-test scores of the experimental groups and the control group showed that there was a significant difference in the use of a variety of vocabulary in writing in favor of the experimental groups indicating that journal writing- whether in-class or out-of class- enhances students' use of a more varied vocabulary in their writing. A comparison between the writing self-efficacy questionnaire of the in- class and the out-of-class groups before and after the intervention showed that there was no significant difference in the writing self-efficacy of the two groups. 7. A comparison between the writing self-efficacy questionnaire within the in class group and the out-of-class group before and after the intervention showed that there was a significant difference indicating that journal writing whether in class or out-of-class promoted writing self-efficacy of grade 11 students. 8. After the ten week period, the researcher interviewed 20 students from the two experimental groups to see their impression about the intervention and consider their recommendations to improve the implementation of journal entries. The students' overall opinions were positive about the intervention of journal writing