Abstract: In MOC (method of characteristics), angular flux is calculated by sweeping over all the tracks densely arranged in the district. The sweeping process is independent of material distribution and geometry. As a result, the capability of treating different kinds of geometry structures of MOC is limited by the capability of geometry modeling modular. The geometry modular of the code MOCP (method of characteristics for high temperature pebble-bed reactor) was developed based on CSG (constructive solid geometry) method. The geometry modular of MOCP is capable of describing the stochastic distributed pebble-bed. The meshing method of spherical fuel was studied. The result shows that relative error of k_eff of a critical sphere is less than 0.1% when the critical sphere is divided into more than 30 layers. Furthermore, the 3D ray tracing efficiency is greatly improved by optimizing the geometry tree and MOCP is able to arrange track on the different shapes of the geometries including cube, cylinder and sphere.