This study aimed to investigate the effects of the integration of indirect feedback and concordances on improving grammatical accuracy in Thai EFL undergraduate students' writing and to explore the students' perceptions toward the intervention. Thirty third-year undergraduate students who studied at a government university in Bangkok were the participants of this study. During the experiment, participants were required to perform three writing tasks. In each writing task, they wrote the first draft and then received both direct and indirect feedback. For indirect feedback, participants were asked to look at concordances to help revise their first draft. The pre-test and the post-test were used to assess grammatical accuracy in the students' writing before and after the experiment. Additionally, the questionnaire and the semi-structured interview were used to find out the students' perceptions toward the intervention. The findings revealed that the number of grammatical errors showed a significant decrease in the post-test compared with the pre-test. As a result, the integration of indirect feedback and concordances was seen to be able to improve grammatical accuracy in the participants' writing. Besides, the results from the questionnaire and the semi-structured interview showed that the participants had positive perceptions toward the intervention on improving grammatical accuracy in their writing because they had a chance to learn from their mistakes after receiving indirect feedback and concordances helped them to induce grammatical patterns.