Abstract: The Nb-bearing J55 oil casing steel was prepared by four rolling processes in two kinds.The effect of rolling process on the microstructure and mechanical properties was analyzed.The results show that the microstructure of the steel prepared by the initial high temperature finish rolling process was quasi-polygonal ferrite (QF),pearlite (P) and little granular bainite (GB).The strength and toughness of the steel could reach the specified value of GB/T 19830-2005.The microstructure of the steel obtained by the initial low temperature finish rolling process was acicular ferrite(AF),GB and QF.Its strength and tounghness were increased obviously,and the mechanical properties of the steel were improved.The inclusions were main composed of sulphide and oxide with high melting temperature,which could improve the toughness of the steel through controlling the inclusion content.