The purpose of the study is to investigate the use of if-conditionals in the corpora of NS and NNS argumentative essays. For this comparative study, a collection of American college students'' argumentative essays and a self-compiled corpus of Korean NNS students'' argumentative essays were used. First, the overall frequency of if-conditionals was examined in the two corpora. Next, the use of if-conditionals was coded by semantic categorization. Common features and major problems were also examined in the Korean NNS students'' writing. The results revealed that although the overall frequency of if-conditionals in two corpora was not significantly different, the distribution of if-conditionals in three semantic categories was different. Significant differences between the two corpora were also identified in several semantic sub-categories, revealing some common features and problems that the NNS students had in their use of if-conditionals.