The chlorination of isocyanuric acid (CA) to trichloroisocyanuric acid (TCCA) was performed using sodium hydroxide in slurry phase ('slurry method'). It was investigated that the effects of reaction temperature, mole ratio of sodium hydroxide, pH of the reactant during the reaction, compared to the chlorination of trisodiumcyanuric acid salt in aqueous solution ('salt method'). It was confirmed that pH is a significant factor to affect yield and available chlorine of the product. Two reasons were assumed. One is that local heat formation, which is concerned with increasing pH, causes decomposition of CA. The other is that the reaction mechanism of 'slurry method' differs from that of 'salt method'.