Abstract: Fracture failure of four-channel flange for KQ65 gas production tree was investigated by mechanical properties testing, optical microstructure observation and fractureanalysis. Experimental results show that the cracking of the tree four-way flange is due to improper heat treatment during manufacturing process. Due to the slow cooling rate, the pearlite grains coarsened and polygonal ferrite formed network structure along the grain boundary. So the toughness of the four-channel flange decreased greatly. In addition, the structural change of the four-channel flange neck caused local stress concentration and promoted crack initiation here.