We report on high-performance 4H-SiC-based p-i-n ultraviolet (UV) photodiodes and investigation of the capacitance characteristics. The fabricated p-i-n photodiode exhibits a large UV-to-visible rejection ratio (R266 nm/R380 nm) while displaying a low dark current and a high responsivity at room temperature. Interestingly, even at 450 k, the photodiode presents a high responsivity of 0.15 A/W and high UV-to-visible rejection ratio more than 200. Capacitance-voltage measurements reveal that the 4H-SiC p-i-n photodiode presents strong frequency-, temperature-, and wavelength-dependent capacitance properties. These results indicate that the advances on the 4H-SiC material and the p-i-n junction offer exciting opportunities for important UV detection in a variety of commercial and military fields.