: Subject of this dissertation is the life prediction of a helicopter main rotor shaft. since application of Damage Tolerance concepts on the shaft is not possible for practical reasons such as limitations due to iectability, according to FAR regulation, the safe life approach was used. Because the shaft loading is non-proportional and the material has low ductility, the combinations of the amplitude of alternating shear stress with maximum normal stress on the critical plane should be used as controlling parameter for fatigue damage. Therefore for life prediction findley criterion was applied.The Loading of the shaft, effect of several components like tail plane, tail rotor and hub on the loading was studied. Because applied torque variation with the blades rotation is different from the applied thrust force variation, two separate finite element analyses for each of the loading was done. Because of shaft symmetry, one thirtieth of the shaft that includes a tooth of the spline was modeled. For life prediction according to findley criterion an APDL code was developed in ANSYS software. Because of the linear material behavior, the code uses superposition principle and results of the two finite element analyses to attain stress tensor variation with the blades rotation in each node. The code applies a load factor of 2.5 that ensures us about the most severe manoeuvres. The code uses the minimum circumscribed circle approach to calculate the amplitude of shear stress. The code determines the critical plane according to findley criterion and compares the damage parameter with material specifications to predict the life. The output of this code is life contour of the elements attached to the selected nodes.To find the critical node the life was calculated in extremum of components of stress tensor. The critical node is in the root of spline tooth. The shaft life was calculated 4389542 revelutions equal to 250 flight hours for the most critical manoeuvres in this node. Life contour of the elements around this node was plotted. For plotting fatigue sensitivity in the critical node, the loading was changed with coefficient of 0.5 to 1.5 and fatigue result variation was observed. Fatigue sensitivity analysis implied that if stresses were decreased two times, the life would be infinite and if stresses were increased two times, the life would decrease 66 percents. Replacing the material with AISI/SAE 4340 that has ultimate tensile strength of 125 Ksi and AISI/SAE 4130 with ultimate tensile strength of 117 Ksi leads to life reduction of 99 percents and 55 percents, respectively. Key words: life prediction, non-proportional loading, critical plane approach, helicopter main rotor shaft.