Persistent photoconductivity(PPC) is a deterring factor for the time response of ultraviolet photo-detectors made of polycrystalline diamond films. The existence of PPC can greatly prolong the response time of photo-detectors. In this paper, ultraviolet photo-detectors with interdigital electrode spacings of 20 μm and 30 μm(denoted as "device A" and "device B", respectively) are fabricated on microcrystalline diamond films, and the influences of grain boundary on time response of diamond ultraviolet photo-detectors are discussed. Results show that performances of PPC and photoconductive gain are present in the two photo-detectors, and the PPC is longer and the photoconductive gain is higher in device B than in device A. It is suggested that grain boundaries may induce a shallow level in the bandgap and act as minority carrier trapping centers, leading to PPC and high gain. There are more grain boundaries between the electrodes in device B than in device A, which hence explains the higher photoconductive gain and responsivity as well as more significant PPC in device B than in device A.