The objective of this study is to investigate the characteristics of welded joints on the Zircaloy-4 resistance upset welding for HWR(Heavy Water reactor)fuel rods. To estimate the characteristics of welded joints, the various tests were performed on the test coupons systematically with a wide range of each welding parameters in terms of a tensile test, burst test, knoop hardness test and metallography. Major results obtained in this study are as follows: 1. The tube and machined with 120.deg. projection was the reliable weld joint design for the nuclear fuel rod end cap welding. 2. As the weld current and the amount of upset increased linearly with increasing welding main heat input, it could make an estimate of their variation in accordance with the phase shift control. 3. It was found that an increase in squeeze force has an effect on the upset contour of welded joint because the amount of upset were increased by the change of squeeze force.