Nitride-based flip-chip p-i-n photodiodes were fabricated and characterized. It was found that we could achieve a small dark current of 5times10-10 A at -5 V and a large rejection ratio larger than three orders of magnitude. It was also found that the photodiodes only detect optical signals with wavelengths between 365 and 378 nm. Furthermore, it was found that peak responsivity occurs at around 370 nm with a value of 0.21 A/W at zero bias which corresponds to 70% external quantum efficiency