Sesame oil bleaching by acid-activated rice hull ash is examined. Lovibond color indices of sesame oils before and after bleaching by activated rice hull ash were determined to calculate the bleaching efficiency of the ash, Surface area meter and inductively coupled plasma-mass spectroscopy were employed to determine specific surface areas and average pore diameters as well as metallic contents of the rice hull ashes before and after acid activation, respectively. Effects of the following factors on bleaching efficiency have been studied; acid type, acid concentration, activation time, ashing temperature and ashing time. Experimental results indicate that rice hull ash activated by H2SO4 has relatively higher bleaching efficiency than that activated by H3PO4 or HCL. The increase in acid concentration or activation time did promote bleaching efficiency, though mildly. Acid activation might increase bleaching efficiency of rice hull ash from 0% to 43%. The effect of activation is found to be most significant when the ashing temperature is in the range of 673-773 K or the ashing time is over 1,800 s. The increase in bleaching efficiency by acid activation might be due to the increase of number of active site available for adsorption on the pore surface of the rice hull ash, Replacement of activated clay as an adsorbent to bleach sesame oil by acid activated rice hull ash is found to be promising.