Aspect ratio is an important parameter in the study of flow through non-circular microchannel. In this work, three-dimensional numerical study is carried out to understand the effect of cross aspect ratio (height to width) on flow in diverging and converging microchannels. Three-dimensional models of diverging and converging microchannels with angle 2–14°, aspect ratio 0.05–0.58, and Reynolds number range of 130–280, are employed in the simulations with water as the working fluid. The effects of aspect ratio on pressure drop in equivalent diverging and converging microchannels are studied in detail. At small aspect ratio and small Reynolds number, the pressure drop remains invariant of angle in both diverging and converging microchannels; the concept of equivalent hydraulic diameter can be applied to these situations. Onset of flow separation is found to be a strong function of cross section aspect ratio. The existence of a critical angle with relevance to the concept of equivalent hydraulic diameter is identified and its variation with Reynolds number is discussed. Finally, the effect of aspect ratio on fluidic diodicity is discussed which will be helpful in the design of valveless micropump. Although the effect of flow separation in diverging and flow acceleration in converging passages on deviation in flow behavior is known, such a strong dependence of these parameters on aspect ratio has not been shown earlier. These results help in extending the conventional formulae made for uniform cross sectional channel to that for diverging and converging microchannels.