The effect of electroslag remelting (ESR) on nonmetallic inclusions in an optimal component 42CrMo steel was studied by BX51M optical microscopy and Image Pro-Plus software, and the continuous cooling transformation (CCT) curve was measured on a Gleeble3000 thermal simulation machine. These results show that ESR can reduce the number and size of inclusions in steel samples and the inclusions removal ratio reaches 71. 90%. The CCT curve of 42CrMo steel was plotted according to the thermal expansion curve and microstructure. When the cooling rate is lower, the microstructure is ferrite, pearlite and bainite;but when the cooling rate is higher, there are martensite and a small amount of bainite.