Abstract: Large difference of impact absorbing energy existed among specimens sampling from adjacent positions of a pressure vessel cylinder with material of 16MND5, when impacted at -20 ℃. The reason for the large difference of impact absorbing energy was found out by metallographic examination, fracture analysis, chemical compositions analysis and EDS micro-area compositions analysis. The results show that micro aggregation level of Mo, Si and other elements of specimens with lower impact absorbing energy was more serious than those of specimens with higher impact absorbing energy, especially for Mo element, which resulted in the large difference of impact absorbing energy because Mo and Si could raise the ductile-brittle transition temperature of the steel. So it could be concluded that it was because improper heat treatment process that resulted in the serious micro aggregation of Mo and Si elements and non-uniform of impact absorbing energy.