In this paper, Monte Carlo method is applied to simulate the process of the self-assembly of amphiphilic diblock copolymer with a series of block lengths of the insoluble and soluble blocks. Under the given simulation conditions, the diblock copolymers form spherical micelles in solution. The dependence of the core radii of spherical micelles on both block lengths is obtained and compared with experimental results of Eisenberg and coworkers.