||Microfluidics is essential in many applications, particularly in biomedical and chemical analysis. Understanding the physics of fluids in microsystems will lead to effective and efficient manipulation of fluid flow in micro domain which is expected to be a dominant market in the future. Separation and vortex flow are important flow features in developing control schemes for microfluidic devices. These are explored in pressure-driven gas flow in microchannels with cavities. The effect of a single or multi cavities on the microchannel flow is first explored. With a single cavity in a long microchannel, the change in mass flow rate and pressure distributions is too small to be detected experimentally. However, in a multi-cavity microchannel, although the measured pressure distribution is still essentially the same, the mass flow rate increases almost linearly with the number of cavities. These findings are consistent with numerical simulations which revealed a minute effect on the pressure drop around the cavity region. The flow pattern, normalized velocity and pressure fields within the cavities are the same for microchannel with different numbers of cavity-pair and at different locations. A reduced Reynolds number rRe is used to characterize the flow pattern in the cavity. Both pressure drop and channel height have been varied incrementally to examine the flow pattern evolution. It turns out that on top of the reduced Reynolds number, there exists another control parameter that is purely geometric, i.e. the aspect ratio between the cavity height and width ARch. Three different flow regimes have been identified: (i) fully-attached if the rRe<4 and ARch<2/3, (ii) fully-separated if rRe>100, and (iii) separated flow if 100>rRe>12 and ARch>2/3. Six distinct flow-pattern evolution modes are observed with increasing rRe depending on ARch. The circulation plot Τ ~ rRe and the normalized circulation plot τ̃ ~ rRe illustrate two governing formulae corresponding to two flow regimes. Breaking down the circulation values to its positive and negative contribution has led to a clearer understanding on the flow interaction at cavity interface and its vorticity distribution.