A series of aliphatic polyamide (nylon) sheets with different draw ratios were prepared and their blood-contacting properties examined in relation to the bulk and surface characteristics. Increases in both the molecular orientation and crystallinity were observed with increasing draw ratio. Their surface wettability slightly decreased with draw ratio, indicating less amide linkages at their surfaces by the drawing process. An increase in contact angle hysteresis by the drawing process is thought to be due to the existence of more amorphous chains at the surfaces. The adsorption of albumin and fibrinogen onto nylon sheets was significantly reduced with draw ratio, suggesting the formation of well-established crystalline-amorphous microdomain structures at the surfaces. Blood compatibility of these surfaces was greatly improved with draw ratio in terms of a change in cytoplasmic calcium levels in platelets contacting these surfaces. Thus, it is suggested that the control of crystalline-amorphous microdomain structures by the drawing process is a feasible approach to improve blood compatibility of commercially available semicrystalline polyamides.