After the low-temperature impact test of cylinder block forging 4330， it was found that the test results were unstable， and the impact test datas of some forgings were far below the technical standard requirements. The reasons for intergranular fracture were generally related to overheating， overburning， trace impurity segregation at the grain boundary and two types of temper brittleness. The metallographic examination and scanning electron microscope （SEM） analysis of the forgings were carried out. The results showed that the contents of Cr， Mn and Mo in the coarse grain region were obviously higher than those in the fine grain region， especially the Mn content was higher. According to the analysis of the impact test results， the influence of dendrite morphology on the impact properties was obvious. And the less obvious dendrite， slight dendrite strip and small dendrite mesh had a little influence on the impact properties. Otherwise， the influence was larger.