A modified Monte Carlo method is proposed to simulate the two-dimensional normal grain growth in polycrystalline thin films. With the newly modified method, not only the simulation efficiency is improved but also the simulated time exponent of grain growth attained n = 0.49 +/- 0.01, which is very close to the theoretical value for steady grain growth n = 0.5. Simulation of the complete process of normal grain growth including the steady state is made possible by means of the present method. The grain size distribution in the simulated thin films was found to vary continuously and slowly with time, the gamma and the Hillert functions may be two of the expression forms during its transition, and the latter corresponds to quasi steady grain growth. The so called "self-similarity" of the grain size distribution during the normal grain growth in two-dimensions is also discussed according to the simulation results.