This paper aimed at obtaining a clear understanding of Iranian EFL learners' L2 writing error types. To develop such an understanding, a research question was formulated to see whether there is any significant difference between the participants' language proficiency level and their error types in writing. To this end, a sample version of the structure and reading sections of a paper-based TOEFL test, and a Test of Written English (TWE) were administered to 103 university students majoring in English. The statistical analyses revealed that a) there were statistically significant differences among proficiency groups on overall error types they made in their compositions, and b) frequency of occurrence of error types in each group was different.