Ti/Al/Ti/Au and Ti/A1/Ni/Au ohmic contacts were fabricated on AlGaN/GaN heterostructure under different temperatures of rapid thermal processing (RTP). Since abnormal resistance values were observed during the contact resistance testing, the surface morphology and contact borders of the samples were analyzed to determine the physical mechanism. Such abnormal phenomenon is found to originate from cracking of the AlGaN layer during RTP, flowing of Ti/Al metallic liquid along the crevices, and continuous reaction of the metallic liquid with AlGaN/GaN. Such processes result in abnormal conduction channels. The possible mechanism of the crevice formation was discussed, and the possible solutions to avoid the crevices were proposed.