Three-dimensional numerical simulations have been performed to study the effect of external cavities located on the side walls of a microchannel. Rectangular micro-cavities have been modeled with different heights (5 µm < h < 100 µm) and alignment. A compressible flow solver is used for carrying out the simulations at various outlet Knudsen number (4.44×10^-4 < Kn < 2.07×10^-2) and Reynolds number (0.1 < Re < 50.3) at a wall temperature of 300 K. It is observed that friction factor (f) decreases with increase in cavity height. There exists a critical height (h_c) beyond which decrease in Poiseuille number (fRe) does not occur at a specific Re and Kn. Drag reduction is not achieved by cascading the transverse cavities on the upper and bottom wall of the microchannel with a phase-shift. This study is applicable is designing low flow friction microchannels.