An increasing number of recent studies have applied the insights and methodologies of corpus linguistics to second language (L2) writing pedagogy and research. This study examined the effects of corpus use on how it affects their development of L2 grammatical accuracy Iranian EFL learners in writing. Additionally, the learners' attitude toward using corpus for writing was investigated in this study. Participants were 60 upper-intermediate language learners of English who were divided into two homogenous groups according to the results of an Oxford Placement Test (OPT). Then, all the participants of two groups were given a pretest, the result of which assured the homogeneity of the groups with regard to their writing skill. The first experimental group passed the course in a laboratory and wrote their essays on a computer and checked them by using corpus. In this study Grammarly was used as a corpus tool. The second experimental group used online dictionaries to write. Also, students of the second experimental group wrote their writings conventionally, on paper and it was the teacher who checked their essays. Finally, participants sat for the post-test after the treatment. Findings revealed that there was a significant effect of using corpus on learners' grammatical accuracy in writing. As for the second question, there was a positive attitude toward using corpus. Findings of this study may inspire teachers to benefit from these types of writing activities.