Abstract: MCNP(Monte Carlo N-Particle Transport Code) is a general purpose N-particle transport code, and its input file contains 3-D geometry model. The paper studies the algorithm of automatic conversion from B-Rep to CSG. The algorithm is the basis of developing MCNP aided modeling tool. Based on SHAPIRO's work, and taking into consideration of the features of MCNP geometry model, the paper improves the algorithm on two aspects, constructing sufficient surface set and decomposing cell. The improvement reduces the algorithm's complexity, and removes some redundant decomposition. Based on the algorithm, the software package Mcam(MCNP Automatic Modeling) is developed. It has been used & tested by many users at home and abroad, and the test result is satisfactory.