Abstract: Working on SLA, most of researchers have focused either on the advanced learners or the future achievements of young learners. consequently the young language learners’ early second language writing development has been largely neglected and needs to be taken into a serious consideration that is of the areas in SLA which can be investigated regarding different aspects, the young language learners themselves and the teachers who are teaching them English writing. The present study has investigated the beliefs of Iranian English teachers on the teaching of second language writing as one of the major roots for teaching English Writing.130 Iranian English teachers were selected to participate in this study of whom 65 were teaching to children and 65 were teaching to adolescent. However both have experiences in teaching the other group too. Through a 35 close ended questions designed questionnaire, the study examined the teachers’ belief in teaching writing and the extent to which their beliefs were reflected in their writing practices in their classroom. At first, a comparison was made regarding the same age groups. Then, the obtained data from both children and adolescents groups were compared in order to gain an overall view. The results of data analysis suggest a number of findings First, teachers’ theoretical knowledge do not significantly differ in their beliefs about the “importance of writing”, “Meaning of writing” and “Feedback and error correction” that they should provide in their classes when they are teaching witting. Second, The differences indicates that English teachers of children and English teachers of adolescents believe to adopt different approaches and use quite different practices and activities in their classrooms when they are teaching children and adolescents. Third there seems to be other factors affecting Iranian teachers' belief about writing that were not studied in this research and, therefore, need to be considered by further research. Other factors such as student-related factors, cultural context of school, and educational curriculums may affect teachers’ beliefs.