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GaN metal--semiconductor--metal (MSM) ultraviolet (UV) photodetectors (PDs) with ZrO2insulating layers were successfully fabricated and characterized. It was found that we can achieve a small dark current and large photocurrent to dark current contrast ratio from the proposed devices with the use of ZrO2insulating layers. With a 20 V applied bias, it was found that the leakage current of the fabricated MSM PDs with ZrO2insulating layers was $1.73 \times 10^{-10}$ A. This small leakage current should be attributed to the large barrier height caused by the insertion of the ZrO2insulating layers. We can also achieve a large UV to visible rejection ratio from the PDs with ZrO2insulating layers. Furthermore, it was also found that we can significantly reduce noise equivalent power and enhance detectivity by using ZrO2insulating layers.