This study investigates thermal effects on the flow and pollutant dispersion in urban street canyons. A two-dimensional numerical model with a k–ϵ turbulent closure scheme is developed, and the heat transfer between the air and the building wall or street-canyon bottom is effectively represented by a wall function. For each of seven cases with different aspect ratios (building height/width between buildings = 0.5, 1, 1.5, 2, 2.5, 3, and 3.5), four thermal situations (no heating, upwind building-wall heating, street-canyon bottom heating, and downwind building-wall heating) are considered. In the cases of upwind building-wall heating, one vortex appears regardless of aspect ratio. When the aspect ratio is greater than or equal to 1.5, the upward motion forced by upwind building-wall heating overcomes the downward motion that appears in the cases of no heating. In the cases of street-canyon bottom heating, when the aspect ratio is less than 3, flow patterns are similar to those in the cases of upwind building-wall heating. This similarity is because the maximum temperature axis is shifted toward the upwind side by the horizontal motion. However, when the aspect ratio is 3 or 3.5, the horizontal velocity is not strong enough to shift the maximum temperature axis toward the upwind side. When the maximum temperature axis is located near the center of the street canyon, two counterrotating vortices appear side by side in the lower layer due to the thermal upward motion around the axis, while the vortex in the upper layer is little influenced by bottom heating. With downwind building-wall heating, two counterrotating vortices appear except in the 0.5 aspect ratio case. To a large extent, the vortex in the upper layer is mechanically induced by the ambient wind, while the vortex in the lower layer is thermally induced by downwind building-wall heating. The dispersion of pollutants released at the street level is shown to be quite dependent upon aspect ratio and heat source location. The vortex number and intensity greatly influence the residue concentration ratio (ratio of the total pollutant amount remaining in the street canyon to the total amount of pollutants emitted) by controlling the travel pathway and escape time of pollutants.