A large scale ingot was dissected to study the segregation law of the elements. The influence of composition segregation to the microstructure and mechanical properties of SA508-3 steel was studied by the comparison of three positions on the ingot. Two precipitated phases were approved to be alloyed cementite and molybdenum carbide (Mo2C). It has demonstrated that the middle and upper parts of the ingot were almost the same in compositions except for a slightly difference in the carbon content. The upper part with more carbon included has relatively more carbide precipitation after the performance heat treatment. The bottom part of the ingot has the lowest carbon and molybdenum content, while the reduction in the amount of precipitated carbide was not observed. On the contrary, fine needle-like Mo2C are extensively distributed in the matrix of the bottom part besides a variety of coarse cementite rods. And the data of Energy Dispersion Spectrum (EDS) mapping has suggested that precipitation of Mo2C tends to bring the segregation of impurities. Many large inclusions were found in the bottom part of the ingot, which were considered to be the main reason for the strength loss of this area.