Monte Carlo calculations have been carried out for aqueous solutions of acetonitrile (AN) at 298.15 K and 1 atmosphere within an NPT ensemble. Water-water potential used is TIP4P and water-AN and AN-AN potentials are from OPLS. Two pure liquids and 12 aqueous AN solutions of different molar ratio, with emphasis on water-rich region, have been used. Excess enthalpy and volume of mixing are calculated from the results and found to agree semi-quantitatively with experimental data. It is found that mixing is not uniform for the whole composition ranges, clustering of AN is most clearly observed, and head-to-head type dimer formation is suggested in dilute aqueous solution region of AN. This is confirmed by our RISM study described in a companion paper (J. Chem. Phys., 99, 6935 (1993)).