The influence of pit defects on AlGaN surface and dislocation defects in GaN buffer layer on the current collapse of MOVPE-grown AlGaN/GaN high electron mobility transistors (HEMTs) is studied in this paper. Pulsed gate voltage measurements show that the surface pit defects result in gate lag current collapse and increased of source/drain resistance. And the more pit defects exist, the more obvious current collapse and increased source/drain resistance are observed. Pulsed drain voltage measurements show that the drain lag current collapse, which is almost unaffected by the surface pit defects, can be associated with the dislocation defects in GaN buffer layer. Our experimental results indicate that pit defects on AlGaN surface and dislocation defects in GaN buffer layer can be one of the origins of gate lag and drain lag current collapse, respectively.