This paper investigates urban canopy layers (UCLs) ventilation under neutral atmospheric condition with the same building area density (Ap = 0.25) and frontal area density ()V = 0.25) but various urban sizes, building height variations, overall urban forms and wind directions. Turbulent airflows are first predicted by CFD simulations with standard k-e model evaluated by wind tunnel data. Then air change rates per hour (ACH) and canopy purging flow rate (PFR) are numerically analyzed to quantify the rate of air exchange and the net ventilation capacity induced by mean flows and turbulence. With a parallel approaching wind (0 = 0), the velocity ratio first decreases in the adjustment region, followed by the fully-developed region where the flow reaches a balance. Although the flow quantities macroscopically keep constant, however ACH decreases and overall UCL ventilation becomes worse if urban size rises from 390 m to 5 km. Theoretically if urban size is infinite, ACH may reach a minimum value depending on local roof ventilation, and it rises from 1.7 to 7.5 if the standard deviation of building height variations increases (0%-833%). Overall UCL ventilation capacity (PFR) with a square overall urban form (Lx = Ly = 390 m) is better as 0 = 0 than oblique winds (0 = 15, 30, 45), and it exceeds that of a staggered urban form under all wind directions (0 = 0 -45), but is less than that of a rectangular urban form (Lx = 570 m, Ly -= 270 m) under most wind directions (0 = 30 -90). Further investigations are still required to quantify the net ventilation efficiency induced by mean flows and turbulence.