Partially aminated poly-N-vinylformamides (APNVF) were prepared by the hydrolysis of PNVF and used as the retention aid of rosin size. The dual retention aids system, consisting of this modern polymer and aluminum sulfate (alum) for neutral-alkaline paper sizing using acid rosin sizes, was evaluated by experiment. The results indicated that APNVF was very effective and a small amount of the polymer used together with alum considerably increased the size retention and sizing degree of paper under neutral-alkaline conditions. The cationic charge density of APNVF significantly influenced the sizing efficiency of the rosin sizes. Furthermore, the retention of alkaline filler CaCO3 and paper strength were improved by the polymer addition. It is clear that the polymer can be used as a multifunctional additive for papermaking.