Online grammar checkers have increasingly been used in writing instruction to detect and remove English as a foreign language (EFL) learners' grammatical errors. This study investigated the effects of the automated writing evaluation software "Grammarly" on Iranian EFL learners' article errors which are common in EFL writing. Ninety Iranian students majoring in English language and literature were selected through convenience sampling. They were divided into three groups, with one receiving "Grammarly" and teacher feedback (experimental group 1), one receiving "Grammarly" feedback (experimental group 2), and one receiving teacher feedback (control group). Both descriptive and inferential statistics were employed to analyze the data. In addition, one-way ANOVA, frequency analysis, and thematic analysis were used to analyze the data from pretests/posttests, questionnaires, and interviews. The results showed that the "Grammarly" and teacher feedback group outperformed the other groups in the post-test. This study recommends that EFL teachers consider incorporating online grammar checkers into their EFL writing courses as writing assistant tools.