Tip deformation effect on nanoindentation hardness was analyzed by finite element method. The tip deformation under indentation was characterized by the tip radius fluctuation and was found to be dependent on the properties of the testing material. Calculation of contact area under loading successfully showed the dependency of tip deformation on the yield strength and the elastic modulus. Therefore, a calibration equation could be derived as a function of yield strength and elastic modulus to obtain the correct hardness without containing the error-in-hardness due to the tip deformation. (c) 2005 Elsevier B.V All rights reserved.