A systematic and careful experimental study of the friction factor in the transition region for single-phase water flow in mini- and microtubes has been performed for 12 stainless-steel tubes with diameters ranging from 2083 mu m to 337 mu m. The pressure drop measurements were carefully performed by paying particular attention to the sensitivity of the pressure-sensing diaphragms used in the pressure transducer. Experimental results indicated that the start and end of the transition region were influenced by the tube diameter. The friction factor profile was not significantly affected for the tube diameters between 2083 mu m and 1372 mu m. However, the influence of the tube diameter on the friction factor profile became noticeable as the diameter decreased from 1372 mu m to 337 mu m. The Reynolds number range for transition flow became narrower with decreasing tube diameter.