In this paper, illuminance distribution is calculated by the Monte Carlo simulation. In comparison with other methods, it has simpler arithmetic process. Light is assumed to be of particle nature. It has particular direction taken from a random function. Paths of each particle are calculated, and the final absorbing points are found. repeating this process, the statistical illluminance distribution is determined proportional to the number of particles striking each wall. For a better result and graphic output, each wall surface was divided into many small ones.