An algorithm to parallelize the Metropolis sampling was devised and applied to the Monte Carlo simulations using quantum mechanical calculations (QM/MC simulations). As a 4 CPU calculation needs 670000 s to finish the same QM/MC simulation with the parallelized program, the present algorism reduced the computational time to almost half as long as that of one CPU calculation. The parallelized QM/MC simulation was applied to Diels-Alder reactions between methylvinylketone and cyclopentadiene. The QM/MC (B3LYP/6-311++G**//MP2/6-31G*, PM3) calculation gave averaged activation energies of 12.7, 13.8 and 15.2 kcal mol (1) in aqueous, methanol and propane solutions. These results are well consistent with those observed. (C) 2008 Published by Elsevier B.V.