A novel Monte Carlo (MC) simulation method using Extended Inter-molecular Potential (EIMP) for liquid crystal (LC) systems is reported, The EIMP is calculated, prior to the MC calculation, as interaction energy between two molecules that are located in various configurations. The MC method using the EIMP enables to carry out a calculation of even a system within many molecules in short CPU time. It is applied to the systems that consist of 750-1000 molecules of typical LCs (4-n-alkyl-4 ' -cyanobiphenyls; nCBs). Several phases are successfully reproduced and order parameters of them agree with the experiments.