Measuring fractal dimensions has become a common practice for describing structural properties of porous media. The present paper studies computing method of fractal dimension by using the image storing principles on computer. As an example of its application on nano particles surface structure, by applying Matlab, the binary image and matrix about the surface pore were investigated by processing and analyzing the Scanning Electron Microscopy (SEM) images of nanoparticles. In accordance with the nanoparticles characteristics of binary digital images, have been minutely discussed based on the fractal theory that the principle and method of calculating Fractal dimensions (D) by use of box-counting dimension.