Abstract: Monte Carlo simulation techniques were used to study the microscopic structure of supercritical binary mixtures of methanol-hexane of different solution densitise.The TIP potential energy parameters developed by Jorgensen were applied to model the methanol-methanol,hexane-hexane and methanol-hexane interaction.The stochastic boundary condition was adopted in near-critical region,and compared with the results of periodic boundary condition.The results showed that significant aggregation at near-critical density was not only limited to methanol molecules that one would expect to have specific hydrogen bonding,but also to nonpolar hexane molecules.