A hydrodynamics of deionized water flowing through the micro heat sinks with cavities and ribs are investigated experimentally. The Reynolds number ranges from 70 to 320. Using micro particle image velocimetry (Micro-PIV) and numerical simulation, velocity and vector fields are both obtained. The comparative results show that the distribution of centerline velocity changes periodically, which shows good agreement between simulations and experiments. Moreover, the vortices easily form in the cavities zones if the kinetic energy and negative pressure gradient exhaust along the flow direction. The physical mechanism of heat transfer enhancement is explained from the view point of fluid flow. The main reasons can ascribe that the heat transfer area and flow turbulence increase due to the existent of cavities and ribs under small Reynolds number; the secondary flow is helpful for mixing the cold and hot fluid in cavities zones as well as the increased turbulence in ribs zones under high Reynolds number. (C) 2016 Elsevier Ltd. All rights reserved.