The aim of this paper is to study the applicability of the theory of micropolar ﬂuids to modelling and calculating ﬂows in microchannels depending on the geometrical dimension of the ﬂow ﬁeld. First, it will be shown that if the characteristic linear dimension of the ﬂow becomes appropriately large, the equations describing the micropolar ﬂuid ﬂow can be transformed into Navier-Stokes equations. Next, Poiseuille ﬂows in a microchannel is studied in detail. In particular, the maximal cross-sectional size of the channel for which the micropolar eﬀects of the ﬂuid ﬂow become important will be established. The experimentally determined values of rheological constants of the ﬂuid have been used in calculations.