Effect of washing conditions on the desulfurization and demineralization from petroleum coke were investigated in the leaching process with molten caustic. Washing the petroleum coke with water could remove 99.4% of sulfur, but the ash content increased more than twice due to the remaining sodium in the coke matrix. Caustic could exclude mineral matter such as Si, Al and V which form water soluble sodium salt, but the basic inorganic(Na, K, Fe etc.) could be excluded only by the acid treatment. Because the removing sulfur reacts with coke to form more stable inorganic sulfur compounds, the total sulfur must be expressed by the sum of the sulfur analyzed by the sulfur analyzer and by the ash analyzer. If the acid treatment was conducted with low acid concentration, the decrease in the amount of sulfur content in ash was not satisfactory and needed 0.3 N acid to remove these. Surface area increased from 3.8 to 1,200m²/g after treatment with caustic, and it was affected by acid treatment that the maximum surface area appeared at the acid concentration of 0.3 N.