[Objectives]  Field experiments and surveys were conducted to evaluate the efficacy of using the parasitoid Microplitis mediator to control Helicoverpa armigera in corn and pepper fields. [Methods]  Mass reared M. mediator cocoons were released when eggs of H. armigera were at peak abundance in fields and the percentage of larvae parasitized and field control efficacy were determined. [Results]  A single and repeated release of 7500 laboratory-reared M. mediator cocoons per hectare in cotton fields resulted in 51.22% and 60.35% parasitism of third generation H. armigera larvae, respectively and control efficiency of 68.42% and 77.63%, respectively. In pepper fields, parasitism of second generation H. armigera larvae ranged from 65.68% to 73.30%, and control efficiency was up to 80%. [Conclusion]  The parasitoid M. mediator can be used as a biological control agent for H. armigera in the IPM of corn and pepper fields.