Abstract: Finite element method (FEM) was used to explore the nanoindentation process of several materials under Berkovich indenter. The simulated nanoindentation results with different tip radii were compared with experiments. A semi-empirical relation which depicts hardness with tip radius and indentation depth was derived by theoretical analysis. Results show that the measured hardness with blunt indenter increases with the tip radius and decreases with the depth of indentation. As the tip radius increases, the variation on indentation size effect becomes much more prominent. The results by theoretical analysis and FEM simulation are in good agreement. The analyzed results indicate that the tip effect in nanoindentaion is caused mainly by the underestimation of the contact area as the tip radius increases. Further results show that it is inaccurate to treat the nonideal tips as the ideal tips in theoretical analysis when the tip radius is large (R>60 nm) and the indentation depth is shallow (h≤60 nm) simultaneously, and that the hardness values tend to be overestimated.