In this numerical study, three dimensional laminar flow, heat transfer and other thermal characteristics of a microchannel heat sink, consisting of seven isosceles triangular microchannels, have been investigated. For this purpose, conduction in the solid parts has been considered and two different horizontal inlet/outlet (I-type) and vertical inlet/outlet (U-type) arrangements have been considered. Simulations have been performed for a constant heat flux of 125 kWm-2 entering from the substrate. In previous studies flow of water in rectangular microchannles has been considered, but in this study CuO-water nanofluid has been utilized. The effects of the Brownian motion of nanoparticles and variation of thermophysical properties of the nanaofluid with termperature have been considered and their importances studied. The results show that with increasing pressure drop, the heat sink performance in terms of heat transfer, thermal resistance and uniform temperature distribution at subtrate improves for the two nominated arrangements. Also increasing the volume fraction to 2% improves the heat sink performance, but as it increases further the thermal resistance and the non-uniformity of temperature at the bottom plate enlarge with no heat transfer improvement. Making comparison with the results of the previous studies on the effect of inlet/outlet arrangement proves that the thermal performance is affected by both of the inlet/outlet arrangement as well as the shape and geometry of the microchannels. For the heat sink of this study with triangular microchannels the performance of the I-type arrangement is better than the U-type arrangement.