Aims: To identify the risk factors of posterior iris synechia formation after phacovitrectomy with posterior chamber single-piece acrylic intraocular lens (IOL; Acrysof® SA60AT) or 3-piece acrylic IOL (Acrysof® MA60BM) implantation. Methods: We selected 153 eyes of 153 patients treated by phacovitrectomy between March 2000 and August 2006 and retrospectively reviewed the relationship between iris posterior synechiae and various factors, such as IOL type, tamponade, preoperative iris posterior synechiae, major indications for surgery, axial length and preoperative anterior chamber (AC) depth. Results: IOL type (p = 0.0447), C<sub>3</sub>F<sub>8</sub> gas tamponades (p = 0.0074), preoperative iris posterior synechiae (p = 0.0001) and postoperative AC fibrin deposition (p = 0.0303) were significantly associated with postoperative iris posterior synechiae by multiple logistic regression analysis. The odds ratio (OR) for single-piece IOL versus 3-piece IOL was 2.658 (95% confidence interval, CI: 1.010–6.993), the OR for C<sub>3</sub>F<sub>8</sub> gas versus balanced salt solution was 4.051 (95% CI: 1.455–11.281), for the presence of preoperative iris posterior synechiae 12.868 (95% CI: 3.511–47.165), and the OR for postoperative AC fibrin deposition was 6.012 (95% CI: 1.186–30.468). Conclusion: Preoperative iris posterior synechiae, C<sub>3</sub>F<sub>8</sub> gas tamponade, postoperative AC fibrin deposition and the single-piece IOL could increase the rates of iris posterior synechiae after phacovitrectomy.