Self-selecting of the material has been the arena of discussion by the researchers of L2 pedagogy. While some believe that it can be effective, others believe that it is detrimental to L2 learning. Hence, the purpose of this study was to investigate the effect of self-selected and teacher assigned writing prompts on the writing complexity, accuracy, and fluency of Iranian EFL learners at beginning, intermediate, and advanced proficiency levels. The theoretical aspects of the current research were founded based on Theory of Reasoned Action (Ajzen & Fishbein, 1980) and Choice Theory (Glasser, 1998). Given that, 52 Iranian EFL learners (beginning N = 19, intermediate N = 16, advanced N = 17) participated in this study. Each student was asked to write about two writing prompts: one selected by the students and the other by the teacher. Using relevant indexes, we measured writing complexity, accuracy and fluency with regard to the two writing prompts. The results indicated that there was a statistically significant difference between the writing complexity, accuracy, and fluency of L2 learners when they wrote about their own self-selected prompts and when they wrote about the teacher assigned ones. The results also revealed that L2 learners’ writings were more complex, accurate, and fluent when they wrote about their self-selected prompts. The findings of this study can have some implications for L2 writing instructors and test designers.