Reverse characteristics of diamond PiN diodes were studied under high reverse bias for high voltage applications. We have fabricated vertical diamond PiN diode structures consisting of a phosphorus-doped n-type diamond layer and an undoped intrinsic diamond layer on a heavily boron-doped p-type diamond substrate with Au/Pt/Ti Ohmic contacts on both the top and the bottom surfaces. Temperature dependent current–voltage (I–V) and capacitance–voltage (C–V) measurements were carried out to reveal the diode characteristics. The fabricated PiN diodes exhibited a sharp and repeatable reverse breakdown with positive temperature dependence indicating an avalanche mechanism. The obtained maximum breakdown field was 2.3 MV cm−1 (920 V) which is comparable to the ideal dielectric breakdown field of SiC. The reverse breakdown field and the leakage current were correlated with the hillock density. This research indicates that robust diamond power devices with high breakdown reliability can be achievable after improvement the quality of the intrinsic layer.