In 1986, the International Association of Classification Societies published requirements concerning the calculations of marine diesel engine crankshafts. Since then, only one of the IACS member Societies has adopted the requirment in its Rules, the other member Societies still use their own method of calculations. In this paper, the author analyzes the load on crankshaft and use the basic theory and practical assumptions to establish a new calculation formula. A brief discussion is also made about the IACS published requirements.