Microposts are utilized to enhance heat transfer, adsorption/desorption, and surface chemicalreactions. In a previous study [Yeom et al., J. Micromech. Microeng., 19, p. 065025 (2009)],based in part on an experimental study, an analytical expression was developed to predict thepressure drop across a microchannel ﬁlled with arrays of posts with the goal of fabricatingmore efﬁcient micro-total analysis systems (mTAS) devices for a given pumping power. In par-ticular, a key ﬁgure of merit for the design of micropost-ﬁlled reactors, based on the ﬂow re-sistance models was reported thus providing engineers with a design rule to develop efﬁcientmTAS devices. The study did not include the effects of the walls bounding the microposts. Inthis paper, a three-dimensional computational ﬂuid dynamics model is used to include theeffects of three-dimensionality brought about by the walls of the mTAS devices that boundthe microposted structures. In addition, posts of smaller size that could not be fabricated forthe experiments were also included. It is found that the two- and three-dimensional effectsdepend on values of the aspect ratio and the blockage ratios. The Reynolds number consid-ered in the experiment that ranged from 1 to 10 was extended to 300 to help determine therange of Re for which the FOM model is applicable.