Localized surface plasmon-enhanced diamond UV photodetectors with different Ir nanoisland arrays had been successfully fabricated. Three different Ir nanoisland arrays with the diameters of 20(D20), 30(D30) and 40(D40) nm were deposited on the diamond surface, respectively, and their height was 10 nm. Ti/Au metal electrodes were patterned on Ir nanoisland arrays area and bare diamond surface to form UV photodetectors. According to photoelectric performance WA results, the responsivities of diamond photodetectors with Ir nanoislands were far greater than that of diamond photodetector, and the D40 showed the highest photocurrent and responsivity at 210 nm, and the D20 indicated the highest UV/vis rejection ratio. The results of simulated extinction spectra exhibited three resonance peaks at UV and visible region for each sample, indicating that D20 had the highest ratio of UV/vis, which was coincide with experiment results.