This research was conducted to study changes in functional groups after oxidation of porous carbon synthesized from palm kernel shell and their effects on the performance of material for an electric double-layer capacitor (EDLC). Porous carbon was prepared by pyrolysis of palm kernel shell at a temperature of 800 °C and steam activation. Surface modification was conducted by oxidation porous carbon using hydrogen peroxide (H2O2). Properties of material were characterized using N2-sorption analysis, scanning electron microscopy (SEM), and Fourier transforms infrared spectroscopy (FTIR) analysis. Measurement of biomass-based porous carbon as an electrode for EDLC was carried out using cyclic voltammetry and galvanostatic charge-discharge methods. The test was conducted using a three-electrode system, with carbon as the working electrode, Ag/AgCl as the reference electrode, Pt as the auxiliary electrode. The electrolyte used was 1 M H2SO4 solution. The results showed that oxidation of porous carbon using H2O2 lowers the specific surface area but increases oxygen functional groups in the carbon surface. The results on testing the performance of EDLC, surface-modified carbon showed better EDLC performance of 5-7 times higher compared to carbon before oxidation.