Abutment screw loosening of implant restorations is a common problem in the treatment of dental implant. The purpose of this study was to calculate stress and preload from the elongation measurements and to determine maximum tightening torque without plastic deformation of the screw. The length of each gold alloy UCLA screw was measured after tightening to the manufacturer's recommended torque of 32 N-cm. Similarily, titanium UCLA screws were measured after tightening to the manufacturer's recommended torque of 20 N-cm. Loosening torque was also measured after tightening to 32 N-cm torque for gold alloy abutment screws and 20 N-cm for titanium abutment screws. The results were as follows ; 1. There was a regressive relationship between screw elongation and tightening torque (gold alloy : r2 = 0.987, titanium : r2 = 0.978), and the mean preload calculated from elongation measurements was 501.11 +/- 26.85 N (gold alloy) and 399.43 +/- 7.61 N (titanium). 2. Stress calculated for the gold alloy and titanium screws at maximum recommended tightening torque was less than 60% of their respective yield strengths and within the elastic range. Maximum tightening torque without plastic deformation was 61 N-cm(gold alloy) and 39 N-cm(titanium). 3. For titanium screws, there was a significant difference between loosening after trial 1 and loosening after trials 2 to 5 (p<0.05). No statistically significant difference was seen in mean loosening torques between the first and subsequent trials for gold alloy screws.