This study compared the effects of two kinds of timing of form-focused instruction (FFI) on learners’ grammar acquisition across different proficiency levels. Altogether 126 EFL students from a Chinese junior middle school were assigned to three groups: isolated FFI group, integrated FFI group and control group. All students from each class were further divided into high and low proficiency levels. All students finished pretest, immediate posttest and delayed posttest of an error correction test. The results showed that both isolated and integrated FFI had positive effects on learners, especially on those high-proficiency learners. On the whole, isolated FFI had advantages over integrated FFI. Specifically, isolated FFI produced better immediate and delayed effects on high-proficiency learners than integrated FFI. And for low proficiency learners, there was no significant difference between the immediate effects of the two kinds of timing, but the isolated FFI showed better delayed effect.