The molecular structures of polyethylene glycol (PEG) and chitosan (CS) are illustrated as follows:(2) PEGThe The intermolecular interactions between these two polymers were studied by viscometry with a thermodynamic parameter alpha, which was first proposed by Sun et al. The weight additive rule of the intrinsic viscosity of polyblend relating to the values of each polymeric constituent was attested to with PEG/CS polyblend. The calculation formula of Huggins coefficient for polyblend, k(m), was theoretically deduced, and a very simple expression of alpha was obtained. First, the values of alpha for PEG/CS blends with different PEG molecular weight were estimated from the experimental viscosity data of the polyblends with different mixed ratio. According to these values of alpha, it can be predicted that an attractive interaction exists between the molecule of PEG and that of CS. Second, the viscosity of CS was measured in pseudo-solvents (PEG dissolved in 0.01N sodium chloride aqueous solution) with different PEG concentrations. From these viscosity data, the values of cross Huggins coefficient are calculated to be all larger than the values of the Huggins coefficient both for CS and for PEG. On the revised cu criterion, the dissimilar molecular interaction in PEG/CS polyblend is demonstrated to be attractive too.