Mechanical properties of line pipe steels used in the deep sea or in the severe cold climate depend on alloying elements and manufacturing processes and many efforts have been made to enhance the properties of the line pipe steels. In this study, for systematic approaches to the process design of line pipe steels, its phase diagram was calculated using a Thermo-CalcTM program. The calculations indicated that A3 is around 840oC. Setting the FRT(Finish Rolling Temperature) above A3 appears to be critical to the increase of strength and toughness of line pipe steels by increasing the amounts of acicular ferrite and at the same time by decreasing the precipitation of proeutectoid ferrite. In the case of the FRT below A3, relatively large amounts of proeutectoid ferrite are precipitated from the matrix with the carbon contents of the austenite phase around ferrite being enriched.