J-R fracture resistance tests were performed at the temperature range of RT-288℃ for three kinds of Korean-made SA508-Gr.3 nuclear reactor pressure vessel steels which were classified by steel refining process. The Si+Al refined steel showed the best J-R fracture resistance while the VCD refined steel showed the worst J-R fracture resistance among three kinds of RPV steels. Microstructural analyses were conducted for the materials to correlate microstructure and J-R fracture resistance. Sub-grain sizes were measured for three steels using the EBSD technique. Sub-grain size, as well as prior austenite grain size, of the VCD refined steel was remarkably larger than that of the other two steels. The carbide size and distribution of the three kinds of RPV steels was investigated through SEM and an image analyzer. The result showed that more and bigger carbides precipitated in the VCD refined steel than in the other two steels. These results matched the previously reported analysis for the relationship between microstructure and ductile fracture resistance.