In this paper it is reported that the behavior under tensile load and the influences of lubricants on fastening of polycarbonate (PC) screw are investigated experimentally as the first step to establish the strength design system of plastic screw. Firstly the tensile strength of PC screw is examined varying the strain rate. As a result, it is clarified that the tensile strength of PC screw using its stress area agrees well with that of cylindrical testpiece similarly to the case of steel screw. Secondly, a new concept of the equivalent residual strain is proposed to define the proof stress as it is difficult to decide the permanent strain after unloading in such viscoelastic materials as plastics and the stress corresponding to 0.2% equivalent residual strain is defined as the proof stress of PC screw. Finally, as a result of tightening test using some lubricants, it is proved that tightening can be achieved relatively smoothly by using silicon grease as lubricant.