This paper presents Computational Fluid Dynamics (CFD) simulations to evaluate the impacts of the upstream building height and stack location on the dispersion of the pollutant released from a rooftop stack. The simulations are performed with the 3D, steady, Reynolds-Averaged Navier-Stokes (RANS) equations coupled with Realizable k-ε turbulence model and the species transport equation. The flow development and pollutant dispersion are investigated numerically considering four different upstream building height (HB1) to emitting building height (HB2) ratios (HB1/HB2 = 0.5, 1.0, 2.0 and 3.6) and four different stack locations measured from the upwind edge of the emitting building (Xs) under the approaching wind perpendicular to the building faces. The results obtained reveal that: (1) the flow field characteristics around the buildings, especially the vortex flow above the emitting building roof and the vortex pattern between the upstream and emitting buildings, depend significantly on the upstream building height and much less on the location of the short stack; (2) for an upstream building of lower or equal height with the emitting building, the pollutants from a rooftop stack are immediately transported downwind the stack by the longitudinal flow; (3) when a stack is located inside the wake recirculation zone of the upstream building, the pollutants from the stack are swept by the upwind flow towards the leeward side of the upstream building and the distribution of pollutant concentrations is highly sensitive to both the upstream building height and the stack location. For different upstream building heights, suitable locations for fresh air intakes on the sides of buildings are also proposed based on the pollutant distribution patterns obtained.