The radiation effects on counterflow methane partially premixed flames (PPFs), diluted by introducing water (H2O) vapor into the air stream, were investigated numerically using the OPPDIF code and GRI-v3.0. Three radiation models were compared. Adiabatic model (ADIA) simulations were compared with optically thin model (OTM) and weighted sum of gray gases model (WSGGM) simulations. The numerical simulation results for the three models were also compared with the experimental flame structure for two different equivalence ratios, phi=1.5 and 2.5. The ADIA overestimated the maximum flame temperature, flame width, and extinction limit of H2O, compared with the WSGGM and OTM. However, there was only a slight difference in the WSGGM and OTM predictions for the flame structure. The difference in the estimates among the models for the flame speed had a considerable effect on the location of the rich premixed flame and flame width. The radiation effect on the flame structure was smaller for phi=2.5 compared with phi=1.5, due to the low, narrow flame temperature profile and the reabsorption effect of the radiative heat. The difference in the extinction limits predicted by the WSGGM and OTM was small, compared with the ADIA. The extinction limit predicted by each model was, from largest to smallest, ADIA>WSGGM>OTM. The maximum heat release rate for phi=2.5 showed a peculiar behavior, which was closely related to the complicated distribution of the heat release rate of the methane-rich PPF. Additionally, it was found that H2O played an important role in radiation heat loss, as well as reabsorption. The radiation reabsorption effect on the flame structure was small for a small addition of H2O, and this effect became more prominent as the H2O concentration.