The study aimed to investigate the effect of using journal writing method in solving mathematical problems and mathematical thinking on grade nine students in the Sultanate of Oman. The study sample consisted of 51 grade nine female students. The students were distributed into two groups, the first group was an experimental group and consisted of 25 students and Journal Writing method was used as the teaching method. The other group was the control group and consisted of 26 students, where usual teaching method was applied . The study lasted for 4 weeks (7 lessons aweek). The researcher prepared two tests to collect the needed data: a problem solving test and a test in mathematical thinking. The results showed a statistically significant difference between the mean scores of the grades of the two tests and in favar of the experimental group, which indicated the effectiveness of the Journal Writing method in solving mathematical problems and mathematical thinking. Drawing upon the findings, the researcher suggested some recommendations: first, it is important to train teachers to apply Journal Writing method in the class. Moreover, it is recommended to carry out the study on other fields like other thinking types or attitudes towards math and sustainability of learning.