We have synthesized phospholipid polymers containing 2-methacryloyloxyethyl phosphorylcholine (MPC) units as novel blood compatible polymers and have evaluated their interactions with blood components. It was found that in the absence of anticoagulants, blood clotting was delayed and blood cell adhesion and activation were effectively prevented on the MPC copolymer surface. A little amount of protein adsorbed on the MPC copolymer from human plasma was compared with conventional polymers, and the amount was reduced with increasing MPC unit fraction. To clarify the reason for the little protein adsorption on the MPC copolymer, the water structure in the hydrated polymer was examined with attention to the free water fraction. Hydration of the polymers occurred when they were immersed in water. The thermal analysis of these hydrated polymers revealed that the free water fractions in the poly(MPC-co-n-butyl methacrylate (BMA)) and poly(MPC-co-n-dodecyl methacrylate) were significantly larger than those in the poly(2-hydroxyethyl methacrylate(HEMA)). The conformation of proteins adsorbed on poly(HEMA) changed considerably but that on poly(MPC-co-BMA) was almost the same as the native state. We concluded from these results that the proteins are hardly adsorbed and do not change their original conformation on the polymer surfaces which possess a high free water fraction such as phospholipid polymers.