A numerical simulation of an optically pumped, multimode D2O laser was carried out based on Lamb’s semiclassical laser theory. The numerical code includes the dispersion effect of the laser medium (D2O gas in the cavity) in order to simulate the strength of mode coupling accurately. As a result, multiple-longitudinal-mode oscillations around the Raman resonance frequency were obtained that are roughly in agreement with experimental observations. Energy spectra of the D2O laser emission obtained by the simulation were compared with those of the experimental results for various D2O gas pressures. By including the dispersion effect, temporal variations of mode frequencies were calculated. In addition, resonant pumping was examined to obtain spectrally narrow outputs by tuning a frequency of a pump CO2laser pulse to the line center of the absorption band of D2O molecules. ©1997 American Institute of Physics.