Transfection efficiency was the primary goal for in vitro gene delivery mediated by nonviral gene carriers. Here, we report a modified gene transfection method that could greatly increase the efficiency of, and accelerate the process mediated by, 25 kDa branched polyethyleneimine and Lipofectamine (TM) 2000 in a broad range of cell strains, including tumor, normal, primary, and embryonic stem cells. In this method, the combination of transfection procedure with optimized complexation volume had a determinant effect on gene delivery result. The superiorities of the method were found to be related to the change of cellular endocytosis pathway and decrease of particle size. The efficient and simple method established in this study can be widely used for in vitro gene delivery into cultured cells. We think it may also be applicable for many more nonviral gene delivery materials than polyethyleneimine and liposome.