This work numerically studied particle collection efficiency curves as a function of aerodynamic particle diameter by the porous substrate of an inertial impactor. The simulation was conducted for different Reynolds numbers based on nozzle diameter (Re), resistance factors of porous substrate (K) and nozzle diameters of the inertial impactor (W). The results show that average flow velocity inside the porous substrate is low for a particle starting from the position close to the centerline leading to a small inertial force. The collection efficiency curves of the inertial impactor with porous substrate are less sharp for the case of higher Re and lower K than the case of lower Re and higher K. This phenomenon occurs because more air penetrates into the porous substrate when Re is large and K is small, resulting in higher particle collection efficiency. The cutoff aerodynamic diameter, dp50, of the impactor with porous substrate decreases with increasing Re, decreasing K and decreasing W. Ultrafine particle loss in the porous substrate can be important. For example, the particle loss of a three-stage cascade inertial impactor with porous substrate increases to 16.5% for particles with an aerodynamic diameter of 0.01 μm due to the diffusion mechanism when K = 568,000 cm-2 and Q = 2 L/min.