An in situ nanoindentation test system was established by combining atomic force microscope (AFM) and scan electron microscope (SEM). This system enabled the indentation experiment using the AFM cantilever with Cube corner diamond indenter, and acquired the curve of load versus displacement. The experimental process can be observed in real time by in situ SEM simultaneously. A mechanical property testing method, based on the AFM cantilever loading curve and the in situ SEM indentation of images analysis, was well developed. The hardness and elastic model of the plastic thin film were obtained by measuring the maximum loading during the indentation and the residual area of the indentation by measuring SEM image. This method was demonstrated by conducting the indentation experiment on the sliver film deposited on the Si substrate using magnetron sputtering. Furthermore, the experiment conducted by the Nanoindenter G200 was presented here for comparison. The results show that the in situ AFM indentation possesses higher resolution of loading and displacement, and can achieve the indentation experiment scaled from nN to mN of the applied load. The hardness of the plastic thin film can be obtained by measuring the residual area of indentation. The pile-up effect of the soft film deposited on the hard substrate, which exists in the Oliver-pharr method, can be reduced by SEM image analyses of residual area and the calculation has a higher testing?accuracy?and?reliability as well