Abstract: Microstructure and properties of high carbon chromium steel with 4wt% aluminum after annealing treatment were studied by means of optical microscope, scanning electron microscope and hardness tester, and compared with those of Al-free high carbon chromium steel. The results show that the density of the high carbon chromium steel with 4wt% aluminum was 7.33 g·cm^-3 which was reduced by 6% than that of the Al-free high carbon chromium steel, the critical transition points increased, two phase region was expanded, the suitable annealing temperature increased by 100℃, and the hardness was higher than that of the Al-free high carbon chromium steel. The Al-free high carbon chromium steel was a hypereutectoid steel, while the high carbon chromium steel with 4wt% aluminum was a hypoeutectoid steel. Aluminum dissolved in ferrite completely and changed the precipitation sequence of second phases, therefore the ferrite phase was formed before carbide phase.