In nuclear power plant, 316 L weldments are widely used in fabricating the primary coolant pipe and in-core components, which usually suffer from very-high-cycle fatigue. Therefore, in this study, the high-cycle fatigue behaviors of 316 L stainless steel with an argon-arc welding joint were studied up to very high-cycle regime by using an ultrasonic fatigue testing system. Microscopic examination indicated that both the base steel and welding seam showed austenitic equiaxed grains. The grain size and micro-hardness values of the welding seam were comparable to those of the base 316 L steel. And heat affect zone was not evidently observed at the weld joint. Fatigue tests showed that fatigue failure still occurred in 107–109 cycles, indicating that the conventional fatigue limit does not exist for 316 L weldment. The fatigue strengths of 316 L weldments were close to those of the base 316 L steel. Scanning electron microscopic examinations revealed that multiple fatigue cracks initiated at specimen surface in the high cycle and very-high-cycle fatigue regime. A modified Murakami’s model was suggested to take into account the effect of surface roughness and Vickers hardness on fatigue life/strength of 316 L weldment. The fatigue strength predicted using the proposed model showed good agreement with the experimental results.