In this study, we investigate the RON degradation in D-mode AlGaN/GaN MIS-HEMTs on a Si substrate via an accelerated step stress at different temperatures. We have observed a three-phase RON degradation behavior, which is highly correlated with a drain bias and back gate bias. First, the RON degradation increases till a peak value when the drain bias increases. Second, when the drain bias increases further, the RON degradation is reduced. Third, the RON degradation slowly increases again. RON degradation is characterized with different temperatures, the results show that 1) high temperature leads to a smaller RON degradation compared to the result at room temperature and 2) high temperature shifts the peak of the RON degradation at a lower drain bias. A possible mechanism that the trapping and de-trapping could occur due to the high temperature and high drain bias is proposed to explain the observed results.