We have investigated the influence of surface modification on the electrical properties of SiC diodes. Schottky diodes (SBDs) as well as PiN diodes were fabricated on n-type SiC substrate with an epilayer, and electrically characterized before and after high temperature annealing, and after removing the surface modified layer, respectively. The devices annealed without graphite cap layer showed ohmic behavior. The surface layer was modified to a conductive layer possibly due to the preferred sublimation of Si species. In order to confirm the existence of modified surface conductive layer, diode was fabricated on the same substrate and electrically characterized after removing 30nm-thick damaged layer by ICP-RIE. The leakage current reduced dramatically, as much as 7 orders of magnitude. The PiN diodes fabricated on the damaged surface layer showed the reverse leakage current and the breakdown voltage of 50mA and 1250V, respectively. While those of the diode fabricated after removing the damaged surface layer were 200nA at the breakdown voltage of 2100V, respectively.