This paper focused on the dispersion of carbon dioxide (CO2) cloud due to large accidental release in urban areas. A three-dimensional urban model was designed and applied to capture the circulation flow and channeling flow in street canyons. The effects of building height, aspect ratio and roof shape on CO2 cloud dispersion were studied by wind tunnel experiment and Computational Fluid Dynamics (CFD) method. Building Reynolds number was calculated to ensure Reynolds number independence and turbulent flow. The results of model evaluation indicate that 100% of the predictions were within a factor of two of the experimental measurements (FAC2), and the relative mean bias (MG) was about 7%. CO2 behaved as a dense gas, whose volume fraction was more than 30% near the ground and about 5% in the upper region, respectively. Only 0.5% of CO2 removed from the canyon was through the top opening. Increasing windward-side building height enhanced vertical recirculation, thus reduced CO2 volume fraction from 22.9% (equal to 422 g/m(3)) to 14.5% (267 g/m(3)). Increasing aspect ratio to 3/2 could take greater amount of air into the canyon through the larger area of the openings, and then reduced CO2 volume fraction to 15.9% (293 g/m(3)). (C) 2019 Institution of Chemical Engineers. Published by Elsevier B.V. All rights reserved.