Abstract: By means of optical microscopy,SEM,tensile testing machine and fatigue testing machine,the microstructure and mechanical properties of N80 and K55 steels for casing drilling were studied and compared.The results show that the microstructure of N80 steel consisted of tempered sorbite,ferrite and a small amount of upper bainite,the microstructure of K55 steel mainly consisted of pearlite and netlike ferrite.There was little difference in tensile strength between the N80 and K55 steels,but the yield strength of N80 steel was 40% higher than that of K55 steel.The tensile fracture dimple of N80 steel was finer and more homogeneous than that of K55 steel,but the elongation of N80 steel was lower than that of K55 steel.The effect of the stress ratio on fatigue crack growth rate of N80 steel was much less than on that of K55 steel,and KIC of N80 steel was higher than that of K55 steel.So the comprehensive mechanical properties of N80 steel was better than those of K55 steel.