Abstract: As a middle voltage auxiliary power source,when nuclear power plant loses all of its outer power supplies,the emergency diesel generation sets will provide emergency power supply to the important safety systems in the plant,to ensure that the reactors may shutdown safely and to protect the important equipments from being damaged.At the end of 2003 and the beginning of 2004,after the disassembling of all 8 diesels in Ling’ao plant,plenty of remarkable cracks were found on 29 of all their 96 pistons.Those serious defects will probably violate the availability of those emergence diesel unitsand endanger the plant safety.This article introduces the analysis process on causes and mechanism how those cracks produced,and a risk analysis on cracks of pistons.