The aim of this study was to investigate the effects of two task types on foreign language written production. Particularly it addressed the issue of how three aspects of language production (i.e. fluency, complexity, and accuracy) vary among two different task types (i.e. argumentative writing task and instruction writing task). One hundred sixty eight intermediate learners of English were randomly selected and divided into two task type groups. Separate ANOVAs were conducted on each dependant variable to see whether there were any statistically significant differences across the groups. The findings revealed that participants in the instruction-task group performed significantly better than those in argumentative-task group in terms of fluency and accuracy. In addition, analysis of complexity measures showed that argumentative essays were produced with more complex language than the instruction essays. Possible explanations are provided and the pedagogical implications of these findings are discussed.