The fundamental understanding and prediction of liquid flow characteristics in microscale are important to control the performance of microfluidic devices. However, fundamental questions about liquid flow characteristics in microscale have not been settled yet and systematical investigation is needed. A systematical investigation on liquid flow characteristics through microtubes with diameters varying from 44.5-1011 m and relative roughness in the range 0.02-4.32% in the Reynolds number range 29-11,644 was performed in this work, using water as working fluid. Experimental results indicated that early transition occurred when the diameter was smaller than 1000 m, the transitional flow characteristics for smooth microtubes differed from rough microtubes and the friction factor in turbulent region for rough microtubes was larger than conventional theory. Moreover, a parameter was proposed to describe the characteristic of microtube. The characteristic parameter was used to calculate the critical Reynolds number and the friction factor in turbulent region for microscale. (c) 2014 American Institute of Chemical Engineers AIChE J, 61: 718-735, 2015