We studied the behavior of particles in the thermal convective flow field induced by heating and cooling of opposed vertical walls experimentally. The object is the effect of the circulating convection on a particle behavior during sedimentation or floatation. Visualization of particle behavior revealed that the thermal convective flow segregated the particle into the hot wall side, resulting in the local re-circulation of particles along the hot wall when the particle density is higher than that of fluid, while the flow segregated the particle into the cold wall side when the particle density is lower than that of the fluid. These segregations are center symmetrical when the absolute values of density differences are the same. In order to investigate the main cause of the segregation, we simulated the particle trajectories in two types of circulating flow: (i) thermal convection and (ii) simple circulation induced by shear. By comparing the particle trajectories in these two flow fields, it was found that the circulating fluid motion causes the segregation, but the thermal convection enhances the segregation. The enhancement must be attributed by the distribution of fluid properties induced by the temperature distribution.