The Ca ion distribution in La1-xCaxMnO3(x≤1/3) has been studied using atomistic simulation. It is found that at low temperatures the doped Ca ions tend to be distributed in clusters with a size of about 6—19?. The stability of the clusters at high pressure and elevated temperature has also been studied, and the calculation results show that the Ca clusters are stable at 3 GPa and 120 K. This chemical phase separation may be one of the causes of the structure and electronic/magnetic inhomogeneity in La1-xCaxMnO3 found in experiments.