To evaluate the efficacy of prophylactic administration of the topical diclofenac 0.1 % on macular thickness in diabetic patients following phacoemulsification and intraocular lens implantation surgery. In a Randomized double-masked clinical trial, 108 eyes of 108 diabetic patients underwent phacoemulsification and intraocular lens implantation surgery were enrolled. Fifty four eyes received the conventional postoperative care with steroid drop, whereas the other group (54 eyes) was given a preoperative diclofenac drop four times daily in addition to steroid drop and continued 6 weeks after surgery. All patients were evaluated by optical coherence tomography (OCT) preoperatively and days 1, 30, and 90. The outcome measures including best-corrected visual acuity and OCT findings were compared in and between both groups. Mean CMT changed from 239 ± 16 and 235 ± 17 at baseline to 249 ± 15 and 254 ± 28 at 3 months in the case and control groups, respectively(p = 0.003). Mean total volume of macula changed from 7.61 ± 0.22 and 7.56 ± 0.3 at baseline to 7.64 ± 0.22 and 7.65 ± 0.31 at 3 months in the case and control groups, respectively (p = 0.820). In comparison to the quadrants thicknesses and mean of them between groups did not reach the significant level. This study showed that there is a beneficial effect of topical diclofenac for prevention of macular thickness increasing following phacoemulsification in diabetic patients.