Modified Demet III and Demet IV processes were studied to regenerate the spent resid fluidized catalytic cracking (RFCC) catalyst by removing metal poisons. A total of 33.1 wt % of V, 89.2 wt % of Ni, and 67.7 wt % of Fe were removed by modified Demet III, and 37.3 wt % of V was selectively removed by modified Demet IV. The results of a microactivity test (MAT) showed that the activity and selectivity of the regenerated catalysts were considerably enhanced. It was proven by the results of MAT, unit cell size of zeolite, and Brunauer-Emmett-Teller surface area and pore volume that the severe condition for the removal of poisonous metals such as sulfidation and calcination at excessively high temperature and washing with only oxalic acid caused destruction of catalyst structure and consequently did not rejuvenate the spent RFCC catalysts though lots of metal poisons were removed. The regenerated catalysts by Demet IV were tested in a Davison circulation riser (DCR), and the results showed that the conversion and gasoline yield of regenerated catalysts were increased without causing any operating problem.