This work studies the particle correlation effect on subthreshold pion production in heavy ion reactions at intermediate energies. It is indicated that this mechanism is able to increase the high momentum component in the nucleon distribution in phase space, consequently increasing the production of pions. By using a modified version of the BUU model incorporating the particle correlation effect, we studied pion productions in the reactions of ¹²C on ¹²C, at the beam energies E/A=84, 74 and 60 MeV/nucleon. The results show that the inclusion of the correlation effect leads to an enhancement of pion production and improves the agreement between the predicted and measured data.