The sizing effectiveness of rosin size with four typical polyamines, polyvinylamine (PVAm), polyallylamine (PAAm), poly(dimethylamino ethyl methacrylate) (PDMAEMA), and polyethyleneimine (PEI), was investigated under neutral papermaking conditions. The polyamines with linear structures were more effective than was PEI with a branched structure. The smaller the side chains of the linear polyamine, the larger was the sizing degree of the rosin emulsion size (RE) with the polyamine. The polyamine with smaller molecular geometric size could retain more rosin size on the pulp and form smaller rosin-polyamine complexes.