Branched polyethylenimine (BPEI) is a cationic polymer that have been used to deliver genes and often served as a -golden standard‖ of non-viral vectors. The transfection efficiency/cytotoxicity profile of BPEI depends on its molecular weight (MW). However, due to the strong positive charge of this polyelectrolyte, it is difficult to determine accurately the MW of BPEI, even if applying sophisticated instruments and complicated protocols. Here, we show a colorimetric assay for the measurement of BPEI MW, based on the aggregation of unmodified gold nanoparticles (Au NPs). We found that the different MW of BPEI (1.25 μg/ml) leads to the different aggregation degrees of Au NPs and the color changes of the solutions. Thus, the MW can be distinguished straightforwardly by visual inspection and determined by spectrophotometry. This novel approach is simple (mixing solutions at room temperature), rapid (within 15 min) and sensitive (only depending on the MW).