The purpose of this study is to reveal the common written mistakes and it's reasons for students who study in institutions of teaching Arabic language to non - native speakers in Sultanate of Oman. They are 36 students (male/female) have been selected randomly which is 35% of the study. A descriptive method has been used to achieve the goal of this study by a test includes 45 questions relate to written errors including expressive errors, spelling mistakes, grammar errors and Morphological errors. This test has been reviewed and approved. The result of this research as follow: 1. Expressive errors are the most common among the written errors in the institutions of teaching Arabic to non-Arabic speakers in the Sultanate of Oman. It is followed by spelling mistakes then Grammar mistakes. Morphological error is the least common errors. 2. There are slight semantic differences for gender variable. However, there are some differences for the education variable in all aspects of common written errors in the institutions of teaching Arabic to non-Arabic speakers in the Sultanate of Oman. According to the study results, the researcher recommended to link the spelling lesson with other branches of Arabic language and learning subjects and to activate the role of the teacher more to follow-up rectification of error and teaching grammar and spelling skills. It is also making the classical Arabic the language of education, providing the curriculum with a series of training and practical applications supported by pictures and drawings, specifying classes for spelling and activating the collaborative learning among students.