The effects of SO2 concentration and particle size on sulphation of paper sludge ash were determined in a thermobalance reactor. The activation energy and pre-exponential factor of sulphation reaction of sludge ash were determined based on the uniform-reaction model. From the X-ray diffraction analysis, only CaSO4 formation is observed without any other sulphation compounds. The outer pore of the sludge ash is not blocked by CaSO4 and the sulphation occurs uniformly throughout the ash. The uniform distributions of CaO and other inert minerals in the ash provide uniform sulphation with good penetration of SO2 into pores of the sludge ash without pore blocking during sulphation of CaO.