One of the most typical and crucial academic writing skills for L2 learners is summary writing. It is a complex activity that requires students’ deep engagement with a text through reading, understanding, paraphrasing and reorganizing and finally constructing a summary in their own words. This study intends to investigate the effect of topic familiarity on Iranian foreign language learners’ summary writing. Its aim is to find out whether there was any significant effect on learners’ summary writing performance with regard to topic familiarity. The participants of the study were 40 female intermediate Iranian EFL learners, who were given instructions for different processes used in summary writing. After each session of instruction, four passages with two different topics familiar and unfamiliar were given to learners in order to be summarized. Two passages had familiar topics; they were related to Iranian culture, so the learners had sufficient background knowledge about them. The two other unfamiliar passages were related to foreign culture, with no preexisting knowledge on the part of the learners. Finally, according to the statistics analysis, it was revealed that learners’ summary writings were significantly affected by topic familiarity. Familiar topics by the activation of learners’ schemata facilitated their understanding and helped them to outperform in summarizing familiar topics than unfamiliar ones. In other words, learners’ familiarity with Iranian culture and their prior knowledge of the content facilitated learners’ reading comprehension as well as their summary writing performance. The result can provide second language teachers with appropriate criteria so that they can improve learners’ writing skills by providing various writing strategies according to learners’ needs and being flexible in selecting teaching techniques, as adhering rigidly to a specific approach will not solve all the forthcoming problems of the learners.