Abstract: A method and a particle cooler for controlling the cooling effect in a circulating fluidized bed reactor. The method comprises regulating heat output from the particle cooler by varying the particle level in the cooler. The particle cooler is mounted in a fluidized bed reactor having a particle flow restrictor with fluidizing means between the flow restrictor and reaction chamber and provides the only particle flow passage between the restrictor and reaction chamber and is provided with internal cooling media tubes dispersed on different vertical levels. Particle flow regulating means is provided for maintaining the particle level within the cooler, which level is adjustable.