An experimental investigation of the liquid flow friction factor and heat transfer coefficient in small diameter channels is presented. Rectangular and circular mini-channels with hydraulic diameters from 0.77 to 2.01 mm were used. Literature in the 1990s showed scattered experimental results and concluded that new physical phenomena in mini-channels could occur However, the present experimental results show that three main causes can explain how different results were obtained in mini-channels compared to macro-tubes: uncertainties oil the channel dimensions, inlet and outlet singular pressure losses, and longitudinal heat conduction. When these elements are taken into account, good agreement is found with standard correlations or theories.