Urbanization in recent years has changed the dynamics of the urban climate in an unfavorable way. Researchers have proposed urban street trees to mitigate some of the adverse effects. This study numerically simulates an experimental study performed on thermal effects of street trees on urban airflow and heat transfer on flat and steep street canyons at three different temperature boundary conditions and two heat flux cases. This is the first stage of a larger project of studying effects of trees in mitigating greenhouse gas emission from urban sites. A steady, Reynolds averaged Navier-Stokes (RANS), κ-ε turbulence model is used in Computational Fluid Dynamics (CFD) simulation environment of typical canyons with and without trees. The buildings and ground surfaces of canyons are simulated at isothermal, moderate heat at 42 ℃ and extreme heat at 82 ℃ condition to observe the heat flux. The buildings are further assigned constant heat flux values of 80 W/m2 and 160 W/m2 to investigate the temperature behavior using atmospheric urban boundary layer profile along wind midline for small and large trees. The canyon ground surfaces experienced higher temperature as compared to building roof surfaces. The effect of buoyancy caused by heated surfaces is observed in the flow. For the isothermal case, trees inclusion reduced the flow circulation in both canyons. For extreme heat case, an increase in vertical velocity component and upward flow is observed in the flow due to the surface heating which indicates that the flow in canyons is dominated by the buoyancy effect.