The pressure drop of gas flows in a microchannel filled with a dense pillar matrix was investigated with specific attention to a pillar shape. Pillars of height 250 µm and aspect ratio of about 10 were etched in silicon using an optimized Bosch deep reactive ion etching process. The pressure drop head-loss coefficient due to compression and expansion of gas at the inlet and outlet of the pillar matrix was estimated to be about 1.4 for an opening ratio of 10. A comparison of friction factor correlations for circular pillar cross-sections agreed rather well with the correlations proposed for the macroscale. Experimentally determined friction factor correlations for several pillar cross-sections for Reynolds numbers in the range of 50–500 are presented. Among the various pillar cross-sections considered, sine-shaped pillars have the lowest friction factor. These pillar structures with low pressure drop but a rather large wetted area can be used quite effectively as regenerative materials enabling the development of microcryocoolers.