The experiments of the hot cracking tendency of the alloys were carried out in the HRS directionally solidified device. Hot cracking tendency of directionally solidified Al Cu and Rene 125 alloys was evaluated with a semi quantitative method. Grain boundaries were observed. Its effect on hot cracking is analyzed and discussed. The analysis result with grain boundary's state is in accordance with experiment result. It is proved that grain boundary's state has important influence on the formation of hot cracks. The effects include weakening the binding energy between grains and compensating solidification shrinking in grain boundaries. The comprehensive influence of grain boundary state on hot cracking tendency of an alloy depends on the distribution and amount of the phases with low melting points in grain boundaries.