This paper presents a review of the flow characteristics in microchannels and our own flow visualization work using a photochromic dye activation technique. The review reveals differences in flow behaviour between micro- and macro-channels, and the need to measure instantaneous velocity profiles to explain these differences. In the experiment, de-ionized water containing a photochromic dye was pumped through a square capillary tube with inner dimensions of 96 μm × 96 μm to yield a Reynolds number of 0.1. A pulsed ultraviolet laser beam was used to create dye traces in the flowing liquid, and images of the traces were recorded with a video camera. Friction factor in laminar flow was also determined from pressure drop measurements at several Reynolds numbers using tap water. The experimental results showed the flow in intermediate-sized microchannels at low Reynolds numbers to still conform to conventional fluid mechanics theory, possibly with slight deviation. The success in obtaining quantitative results with this approach holds promise for further studies of flows in microchannels with smaller diameters.