The effects of surface oxides of carbon black specimen(Vulcan XC-72) on the corrosion behaviour in 100% $H_3PO_4$ at temperatures of 148° and 180℃ have been investigated by using cyclic voltammetry and current decay transient measurements. The surface oxides were identified using Fourier Transform Infrared (FTIR) spectroscopy. Cyclic voltammetry measurement was carried out in the applied potential range from -0.4 to 1.2$V_{SCE}$ with a scanning rate of 10mV$s^{-1}$, and current decay transients in log current vs log time were measured at the constant applied potentials of 0.6, 0.8 and 1.0$V_{SCE}$. FTIR spectroscopy was performed on mixture of carbon black and KBr after the carbon black was subjected to constant applied potentials of 0.2, 0.4, 0.8 and 1.0$V_{SCE}$ in 100% $H_3PO_4$ for 2 h, followed by washing annd drying. From the cyclic voltammetry measurements, the anodic current peak was observed to occur at 0.2 $V_{SCE}$, due to the formation of surface oxides which inhibits the evolution of $CO_2$ caused by carbon dissolution. The results of FTIR spectroscopy showed that the surface oxides consisted of carboxyl and quinone and that the amount of carboxyl decreased with increasing applied potential from 0.2 to 1.0 $V_{SCE}$ while that of quinone almost remained unchanged. The current decay transients characteristics demonstrated the two-staged variation of the repassivation exponents with time for the carbon electrodes exposed to constant applied potentials of 0.4, 0.6, and 0.8 $V_{SCE}$. The value of the repassivation parameter decreased with increasing applied potentials, indicating that below 0.8$V_{SCE}$ the surface oxide facilitated the passivating effect that was however aggravated by dissolution of the surface oxide above 0.8$V_{SCE}$. From the above results, it is concluded that carboxyl formed on the surface plays a greate role in the passivation on the carbon black specimen wich is reduced with increasing applied potential.