To substantiate an aerodynamic equivalence between blade flapping and feathering motions which is used widely in trim and performance calculations of a helicopter, numerical study are carried out using the CFD code based on unsteady 3 D compressible Euler equations with a moving grid system. Numerical calculations are conducted for a lifting rotor in hover and forward flight. By comparing unsteady aerodynamic loads on blade which execute pure flapping and feathering motions at various flight conditions, influences of unsteadiness due to blade motion and compressibility on the aerodynamic equivalence are clarified. In addition, effect of wake structure on the equivalence in hover and forward flight are also examined. It is ascertained that the blade flapping and feathering motions are equivalent aerodynamically both in hover and forward flight under the condition if the blade motion can be approximated by simple harmonic motions.