The present study was conducted to compare the impact of direct and metalinguistic written corrective feedback on Iranian EFL learners' grammatical knowledge. The participants were a convenient sample of students in two intact writing classes. The instruction provided in both groups was similar; however, the students in one group received direct feedback and the students in the other group received metalinguistic feedback in the form of error codes on writing accuracy (i.e., grammar, vocabulary, and punctuation) of their in-class written texts. Moreover, all the students took a grammar test serving as pre- and posttests before and after the treatment. In addition to the computation of gain scores, descriptive statistics and a mixed between-within subjects ANOVA were run to analyze the data. Descriptive statistics revealed that the grammatical knowledge of the learners in both groups developed as a result of the two types of feedback; nonetheless, there was not a statistically significant difference between the students' performance on the grammar test before and after the treatment. Furthermore, although the direct feedback seemed to be more effective in improving grammatical knowledge, no statistically significant difference was found between the two groups' gain scores on the grammar test. Accordingly, it was concluded that either of the feedback types may be employed to effectively develop EFL learners' knowledge of grammar.