Experiments are conducted to investigate fluid flows in converging-diverging microchannels (CDMCs). A new dimensionless number related to channel geometry, Gm, is introduced to combine with the Reynolds number, Re, to characterize the flows. It is found that the new dimensionless number, ReG = Re · Gm, is more appropriate than Re for flow characterization in CDMCs. Flows are laminar for ReG < 40 regardless of the geometry of CDMCs. For laminar flows, the flow resistance model developed in the literature works well. For transitional and turbulent flows, a general scaling law for the flow resistance is developed, which suggests a polynomial dependence of pressure drop on the flow rate. Numerical simulations have also been performed to confirm experimental results.