This study aims to provide a linguistic taxonomy of frequent syntactic-morphological errors in Iranian EFL writings in synchronous and asynchronous modes of communication. It also provides the most frequent and the least frequent errors among EFL students' writings in these two modes. Error Analysis is a procedure used by both researchers and teachers for diagnostic prognostic purposes. It involves collecting samples of learner language, identifying the errors in the sample, describing these errors, classifying them according to their nature and causes and evaluating their gravity (Keshavarz, 1999). The researchers conducted a research study on 63 EFL undergraduate university students' writings. Students had to write their ideas on their daily issues. Afterwards, the researchers tried to rank and categorize their erroneous structures. As we had hypothesized, more errors were found in the synchronous mode of communication than in the asynchronous one; however, when investigating the exact types of errors, we noticed that some categories were different in these two modes of communication. Data analysis revealed that the frequency of error types varied with each mode of communication, this same analysis also showed that highly relevant associations could be established for the participants' errors and writing settings.