In material nanoindentation hardness testing, the hardness will decrease with the indentation depth or peak load increase, i.e. indentation size effect (ISE). There are several models and equations were proposed to describe ISE. But the variables self-inaccurate in these models and equations, it will affect the result trueness. Single crystal silicon was used for nanoindentation experiments, and max depths were obtained from these experiments. Combining Matlab software, residual areas were obtained by atomic force microscopy (AFM). Based on max depth and residual area, a new model—residual area max depth model was proposed for indentation size effect in nanoindentaion hardness. The new model perhaps can understand and describe ISE in indentation hardness better than other models and equations.