In this paper, we propose a novel method to achieve efficient rendering of circular depth of field (DOF) effects. Like most of the existing work, we model the task of DOF rendering as the problem of filtering with spatial-varying circular kernels. To avoid direct calculation which is extremely computationally expensive, we approximate the circular kernels with multiple square kernels tilting at different angles. Integral images are then applied to speed up the task of filtering with square kernels. In order to avoid the problem of intensity leakage and edge discontinuity, not only the integral parts, but also the fractional parts of the disparity are considered in calculating the spatial-varying kernel sizes, i.e., the radii of the circles of confusion (COC). Finally, the circular depth of field effect is derived by combining the results of filtering with square kernels at different angles. Experimental results suggest that the proposed method could produce realistic DOF effects with decent running time.