A Monte Carlo simulation using semi grand canonical ensemble method was applied to examine the condition that precipitation free zone (PFZ) appears in an Al-based composite materials under two step aging. The present simulation uses a fcc lattice with a periodic boundary condition for y and z directions, and a grain boundary existing at the end for x direction. A simple grain boundary segregation case gave a nealy parabolic growth law in PFZ width and the volume fraction of precipitates. Different origin of PFZ, namely, mobility or chemical potential, lead to the opposite effect upon secondary heat treatment.