Technology is advancing in areas such as: electronic industries, high power motors and optical devices that produce high heat flux, a way that conventional methods are unable to eliminate. the heat generated by these devices and requires advancements in cooling methods. There are two ways to improve cooling processes. The first method is to design a new cooling device with smaller channel dimensions that result in improved convective heat transfer coefficient due to increased fluid velocity [1, 2]. The use of a microchannel as a cooling device to dissipate heat from a silicon integrated circuit was first proposed by Tuckerman and Pease [3]. Two important goals in cooling electronics are reducing the maximum device temperature and minimizing temperature gradients on the device surface, which can be achieved through the use of a microchannel heat sink (MCHS ) [4]. The second way to improve the cooling process is to improve the heat transfer properties of fluids [5-9]. Since the thermal conductivity of metallic solids is much higher than the thermal conductivity of fluids, the use of millimeter- or micrometer-sized suspended metallic solid particles in the fluid is expected to improve the thermal conductivity of the base fluid [10]. To overcome the problems such as: abrasion, particle sedimentation, clogging and finally the additional pressure drop along the channel that occurs due to the large particle size, the new concept of nanofluid has been introduced to the first time by Choi and Eastman [11]. Several experiments were performed to suspend various metal and metal oxide nanoparticles in several different base fluids by Choi and Eastman [7, 12–15]. Considering the importance of MCHS in cooling processes, numerical and experimental studies...... middle of paper ......r unless a surfactant is used. Selvakumar and Suresh [22] experimentally measured the convective heat transfer coefficient of CuO/water nanofluid in the turbulent flow of an electronic heat sink. They found that the convective heat transfer coefficient of the nanofluid increases with increasing flow rate and concentration of the nanofluid. The maximum increase in convective heat transfer coefficient was approximately 29%, which occurred at 2 vol% nanofluid. They also proposed a correlation for the Nusselt number in the turbulent flow regime in the microchannel. In this study, the local convection heat transfer coefficient and friction factor of CuO/water nanofluid in a rectangular MCHS were measured experimentally in the laminar flow regime. In these experiments, we focused on the effect of flow rate and nanofluid concentration on the heat transfer coefficient at the channel inlet...