Large amplitude oscillation of cosmic field that may occur right after inflation and in the decay process of weakly interacting fields gives rise to violent particle production via the parametric resonance. In the large amplitude limit the problem of back reaction against the field oscillation is solved and the energy spectrum of created particles is determined in a semi-classical approximation. For large enough coupling or large enough amplitude the resulting energy spectrum is broadly distributed, implying larger production of high energy particles than what a simple estimate of the reheating temperature due to the Born formula would suggest
