The experimental data in the literature have indicated that there are unexplained unusual behaviours in heat transfer and flow in microchannels. In particular, the geometry of the microchannels and Reynolds number alone do not determine the flow regime boundaries and the transition range. The reported experimental data on convection in microchannels are now processed based on the correlation with the Brinkman number, from which the Reynolds and Brinkman numbers at the flow transition points are obtained for a fixed microchannel geometry. In addition to the Reynolds number, the Brinkman number is also discovered to determine the flow regime boundaries from laminar-to-transition and from transition-to-turbulent flow. The transition range Varies due to the difference in the extent of the role played by the Brinkman number in determining the two flow regime boundaries.