Following the previous reports, dealt with in this report is the strength of crankshafts of long stroke marine diesel engines. The authors propose a new method which evaluates the strain of crank pin fillets of an engine under its operating condition, by introducing a new concept 'Response Factor'. The response factor, which is defined as the ratio of strain response to external concentrated force, is classified to 9 types according to kinds of the applied load and positions. The every response factor is obtained effectively by the calculation using F. E. M. zooming model of the pin fillet, and then the strain is calculated through summation of each response produced by the external forces which are evaluated by matrix calculation. Moreover, measurements were carried out on two kinds of long stroke engines to know the stress condition of the crankshaft of such type engines while running. The results show that the pin fillet's strain along radial direction has much effect of the constrained moment from the adjacent throws and the axial vibration. The fact that calculation results by the new method corresponds to measured ones fairly well shows the high reliability and great effectiveness of the method.