In this paper, the AlGaN/GaN HEMT (high electron mobility transistors) with different ohmic contact structures are fabricated, and the effect of different ohmic contact pattern on GaN HEMT electrical properties is studied. A conventional ohmic contact electrode structure and a new ohmic contact structure with a contact hole are fabricated and subjected to rapid thermal annealing (RTA) in flowing N2. After different structured AlGaN/GaN HEMTs are annealed at 750 ℃ for 30 seconds, in HEMTs with a conventional structure ohmic contact still does not form while in the device with ohmic contact holes a good ohmic contact is already formed. Then the surface morphology of different multilayer electrode structures is measured. Comparing Ti/Al/Ti/Au with Ti/Al/Ni/Au, we can conclude that the structure Ti/Al/Ni/Au has a more smooth surface after annealing. After testing the HEMT devices with different structures, higher transconductance and saturation current are found for the devices with ohmic contact holes. But a serious current collapse phenomenon has been observed when the gate voltage is set between 0.5 V and 2 V.