Nozzle is one of key parts in the diesel engine. The cavitation erosion of needle-sealing surface in the pintle nozzle has important influence on the fuel atomization, combustion and power performance of diesel engine. In order to investigate the effect of hardness of needle-sealing surface on cavitation erosion, two kinds of nozzles were selected and operated in S195 diesel engine for 10 hours. One nozzle is heat-treated one which has lower surface hardness; the other with higher surface hardness is real commercial nozzle. The surface appearances of original and operated nozzle-sealing surface were observed by SEM. It was found that the cavitation erosion on the seal surface of nozzle with lower hardness was severer than that of nozzle with higher hardness. However, their wear ways and formations are similar.