It has been known for a long time that LI writers perform differently in different modes of discourse. Despite its importance, there has been no conclusive evidence to shed light on the issue of ESUEFL learners' writing performance across various discourse types especially in EAP environment. Therefore, this research study was designed to investigate differences resulting from the effect of four discourse modes (i.e. Narrative, Description, Explanation, and Argument) on EFL writing skill. The research was conducted in three different phases mainly with university students in Iran. The hypothesised differences were examined in the three dimensions of production (through eliciting compositions), recognition (through cloze tests derived from compositions written in different discourse modes), and finally the learners' attitude towards these types of writing (through questionnaires). The results of the production phase show a statistically significant difference between argument and description but not between narrative and exposition. For the recognition phase, significant differences were observed among all four types of discourse, ranking narration, exposition, description, and argument in order of their difficulty level as cloze tests. At the third stage of the study, an examination of learners' attitudes towards composition prompts was examined which showed their reluctance towards writing in argumentative mode. It can be concluded that argument proved to be the most difficult mode and narrative the easiest in all three phases of this research study. Description stands somewhere in between. Exposition did not follow any consistent pattern and seems to be too broad a category to be considered as a single mode and needs to be narrowed down into more manageable sub-modes. Overall, the findings of this study indicate that the L2 learners at tertiary level behave in the same pattern that has been established for the LI young writers. Recommendations are made to increase the construct validity of writing element of test batteries and to improve pedagogical insights into writing skill.