Recently various pressure vessels like an atomic reactor and plant facilities become more larger and are needed to bear in both very high temperature and pressure condition. And in making such a high pressure vessels the amount of annual usage of 2 $frac {1}{4}$ Cr-1Mo steels which are predominant to resist high temperature high pressure and corrosive circumstances are increasing. But despite of this advantage of 2 $frac {1}{4}$Cr-1Mo steel. when PWHT(post welding heat treatment) is carried out lots of reheating cracks are occur. In this reason it is strongly needed to study and examine the mechanical behavior of welded joints through welding to PWHT process. So in this study welded nozzle of pressure vessel where reheat cracks are frequently occur are selected for analysis the crack-occurrence mechanism.