Abstract: In order to study the changes of microstructure and mechanical properties of 15CrMo seamless steel tube before and after shrinkage deformation, 15CrMo seamless steel tube was formed by cold extrusion. The microstructure,hardness, tensile properties and fracture morphology were observed before and after the steel tube deformation. The results show that the grain deformation occurs in the reduced diameter area, the relative slip between lamellae occurs in the crystal structure, and the dislocation density increases, which leads to the significant increase in tensile strength and hardness, while the plasticity decreases, resulting in work hardening phenomenon. After annealing, the original deformed grains become equiaxed grains, the dislocation density reduces, and the plasticity and strength are improved. By comparing the data of annealed steel tube with that of forged steel tube, the results show that the microstructure and mechanical properties are close to each other.