We obtained the output characteristics in wurtzite Al0.15Ga0.85N/GaN MODFETs with the full band Monte Carlo method. The gate length L g and the channel length LDS in the device are 0.2 μm and 0.4 μm, respectively. In the output characteristics we found a differential negative resistance effect. That is, as VGS is a constant, initially, I DS increases with increasing V DS. When V DS exceeds a certain critical value, I DS decreases with increasing VDS. The analysis for velocity-field characteristics in wurtzite GaN, the distributions of the electric field and the electron velocity in the two dimensional electron gas channel indicates that the differential negative resistance effect of the electron average velocity results in the differential negative resistance effect of the output characteristics. The transient transport also is related to the differential negative resistance effect of the output characteristics. This effect only can be observed in the devices with very short channel.