This study investigated the effect of direct and indirect unfocused written corrective feedback (WCF) on the implicit and explicit grammatical knowledge as well as the writing grammatical accuracy of Iranian EFL learners in their new pieces of writing. To that end, 90 participants selected from a statistical pool of 380 EFL learners in seven private English language institutes were assigned into two treatment groups of unfocused direct and indirect WCF and one control group via random matching technique to receive the intended treatment. Five instruments, namely, Timed Grammaticality Judgment Test, Metalinguistic Knowledge Test, Untimed Grammaticality Judgment Test, Oral Imitation Test, and Writing Test were used to measure the probable learning gains in implicit/explicit grammatical knowledge and the writing accuracy of the participants due to the treatment. In turn, the statistical analyses of MANCOVA, MANOVA, and one-way ANOVA were used to analyze the data. Although no significant difference was identified between the two types of WCF in learning gains, the results showed that these two types of instructional treatment helped improve the explicit and implicit grammatical knowledge as well as the writing accuracy of the participants. Theoretical and pedagogical implications of the findings are discussed.Keywords: Direct/indirect unfocused WCF, implicit/explicit grammatical knowledge, writing accuracy, EFL learners.