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An experimental investigation on the unsteady tip flow field of a transonic compressor rotor has been performed. The casing-mounted high frequency response pressure transducers were arranged along both the blade chord and the blade pitch. The chord-wise ones were used to indicate both the ensemble averaged and time varying flow structure of the tip region of the rotor at different operating points under 95% design speed and 60% design speed. The pitch-wise circumferential transducers were mainly used to analyze the unsteadiness frequency of the tip leakage flow in the rotor frame at the near stall condition. The contours of casing wall pressure show that there were two clear low pressure regions in blade passages, one along the chord direction, caused by the leakage flow and the other along the tangential direction, maybe caused by the forward swept leading edge. Both low pressure regions were originated from the leading edge and formed a scissor-like flow pattern. At 95% design speed condition, the shock wave interacted with the low pressure region and made the flow field unsteady. With the mass flow reduced, the two low pressure regions gradually contracted to the leading edge and then a spike disturbance emerged.