In Europe the water cooled grate system in MSW plant has been often required since 1993,because LHV of waste change higher. Japanese MSW plant also needs same system. So we have carried out some tests for development of water cooled grate. In low level combustion air ratio test, We got high temperature in incinerator and that leads the low level emission of dioxin. We installed water cooled grate in domestic MSW plant, and we confirm steady operation for low level LHV of waste rather than that of Europe.