In order to identify the problematic items in learning English and finding the suitable ways of improving the pedagogical methods and textbooks, the analysis of the errors in the English writing of the high school Persian students according to their age and gender is important. So, the aim of this research was to analyze the errors in the English writing of girls and boys who were learning English in first and third grade of high school. Keshawarz (1994) is the theoretical framework of this research to categorize the errors. In this research, the written errors in the English writing of about 200 girls and boys studying in high schools are analyzed and it tries to answer these questions: 1) what kinds of spelling errors do Persian EFL students have in the writing of English texts and what are their frequency of occurrence? 2) What kinds of differences are there in the English writing of the students according to their age and gender?  3) What are the sources of these errors? There were 739 errors. In this research it has been concluded that the frequencies of spelling errors in the writing of boys are more than the respective girls. Each category of spelling errors consists of several sub-categories. In addition, in this research the sources of the errors have been identified, i.e., the most important sources of errors were Intra-language sources, interferences, communication strategies and the strategies of the foreign languages learning.