Abstract: The friction factor characteristics of rectangular microchannels with aspect ratio ε from 0. 1 to 1. 0 in hy- drodynamic entrance region were investigated by using the Fluent software． The results reveal that the aspect ratio ε and Reynolds number Re have effects on the product (f_app Re) of the apparent friction factor and Reynolds number． The f_app Re of rectangular microchannels decreases as the aspect ratio ε increases，and different effects are presented in the different range of Re． The f_app Re decreases with the increase of Re when Re is less than 300; while the f_app Re varies slightly when Re is more than 300． When the flow is fully developed，the f_app Re remains unchanged and be- comes nearly independent of Reynolds number． The effects of ε and Re on the hydrodynamic entrance length is also studied． It is found that when Re is less than 20，the dimensionless entrance length gradually decreases with the in- crease of the aspect ratio ε． When Re is more than 20，the dimensionless entrance length first increases and then decreases with the increase of the aspect ratio ε，and reaches the peak value as the aspect ratio is between 0. 3 to 0. 4． Based on the obtained results，a new hydrodynamic entrance length correlation for laminar flow was proposed， which may provide guidance for the design and optimization of microchannels．

Key words: hydrodynamic entrance region, friction factor characteristics, the product of apparent friction factor and Reynolds number, the hydrodynamic entrance length