An examination of the structural integrity and defects of a fabricated mercury target vessel for Japan Proton Accelerator Research Complex (J-PARC) spallation neutron source is presented. Ultrasonic testing (UT) and radiographic testing (RT) were employed as nondestructive inspection methods. The mercury target vessel is composed of SUS316L stainless steel and was designed with multi-walled structures consisting of double-guard vessels with thin walls of 3-mm thickness and assembled by tungsten inert gas welding. The mercury target vessel has complex characteristic, and the weld defect for the thin walls is often very difficult to detect using conventional UT techniques. To overcome this barrier, we employed two new UT techniques, namely 1) immersion ultrasonic with a 50-MHz ultrasonic probe and 2) phased arrays ultrasonic with the full matrix capture (FMC) and the total focusing method (TFM). The examination revealed the formation of small defects and cracks wherein the wall thickness was less than 6 mm. Therefore, new UT techniques are useful for evaluating the structural integrity and defects of the new fabricated mercury vessels. The design and the fabrication process of the mercury target vessel was also evaluated and improved in this study. The use of wire electric discharge machining (EDM) in the fabrication process is desirable to reduce the amount of welding and subsequent welding deformation. The roughness and chemical compositions of the processed surface layer by wire EDM were also examined. The oxide layer was perfectly removed by two-step chemical polishing, and the chemical composition of the layer was analyzed using energy dispersive X-ray spectrometry. In addition, the surface roughness was reduced after polishing to enhance fatigue life and minimize internal defects caused by welding.