The authors have studied the effects of metallurgical factors on the corrosion and mass transfer of steels in sodium, using our original sodium loop simulating the secondary coolant system of a proto-type FBR**1**). On the basis of the test results, the authors have developed the computer program of the corrosion and mass transfer. This paper describes the ideas of the above computer program and the computer analysis of the corrosion and mass transfer of steels for the Japanese proto-type FBR 'Monju' in sodium for the plant life. The results are summarized as follows. The metal loss values of 304 and 321 stainless steels and 2 one quarter Cr-1Mo steel are about 10, 10 and 30 mu m, respectively, for the plant life. The total content of carbon gains of 304 stainless steel for the intermediate heat exchanger tubings and 321 stainless steel for the superheater tubings are less than 0. 06 and 0. 02 weight percent, respectively, and the total content of carbon loss of 2 one quarter Cr-1Mo steel for the evaporator tubings is about 0. 01 weight percent, under normal operating conditions for the plant life.