The effect of AlGaN surface traps on breakdown voltage VB and drain current collapse in AlGaN/GaN high electron mobility transistors (HEMTs) were investigated using experimental measurement and numerical simulation. The drain current transient due to surface traps was systematically measured and analyzed, and the activation energy of a surface trap was evaluated as approximately 0.7 eV. Results from the device simulation of VB in HEMTs were in good agreement with the experimental results when assuming surface traps. The results indicate that surface traps increase VB, and induce a crucial current collapse.