When road tankers or pipelines fail in a city, the released gas generates cloud in street canyons. The toxic heavy gas cloud pollutes the environment and harms the living being's life. The concentration distribution and trail of the cloud are very important parameters for the emergency response. The wind tunnel experiment and the Computational Fluid Dynamics (CFD) method were applied in studying the transport and dispersion of carbon dioxide (CO2) plume, which was a typical type of heavy gas, in a long street with an intersection. The numerical simulated results using the SST kappa-omega model were in acceptable agreement with the experimental data. The stationary pre-release flow field in street canyons was characterized by the lee eddy generated on the sharp edges of the building top and sides. In this work, the entire releasing and dispersing process of CO2 can be divided into two stages or two regions. The motion of CO2 cloud was primarily determined by the jet speed and slightly affected by wind in near-source region. The next stage began with the collapse of CO2 cloud. The gravity and wind together influenced the concentration distribution of CO2. Both leakage rate and wind speed had significant impacts on CO2 distribution pattern within an urban street canyon. (C) 2018 Institution of Chemical Engineers. Published by Elsevier B.V. All rights reserved.