In this work, the effects of three parameters including aspect ratio, hydrophobic surface (slip boundary condition) and water/Al 2 O 3 nanofluid in a rotating U-shaped microchannel were investigated numerically. The fluid flow and the convective heat transfer assuming a 3D steady flow with constant and variable properties were simulated with Ansys Fluent. In order to apply the slip boundary condition on the microchannel walls, the additional code was provided and employed in Fluent. At the first part, the effect of aspect ratio on the fluid flow and heat transfer were investigated using the pure water in hydrophilic (no-slip) rotating microchannel. In this part, the aspect ratio of 0.25 to 4, Reynolds number of 400 and rotational speed between 0 and 600 rad/s were employed and velocity profile, temperature profile, pressure drop, Nusselt number and thermal performance coefficient were investigated. The result of this part showed that increasing the aspect ratio in a horizontal microchannel ( ) and decreasing aspect ratio in vertical microchannel ( ) provided an increase in pressure drop and heat transfer. In the second part of this work, the effect of slip and temperature jump boundary conditions in rotating U-shaped microchannel was investigated. Assuming 5 micron slip length, the result showed that the slip condition causes a decrease in the pressure drop and heat transfer. In addition, the thermal performance coefficient of hydrophobic microchannel is higher than that of hydrophilic microchannel. Finally, in the third part, water/Al 2 O 3 nanofluid in a rotating hydrophobic U-shaped Microchannel were investigated by single phase model (constant properties and variable properties) and Eulerian-Eulerian two phase model for volume fraction 1,2.5 and 4 percent. It is found that there is a large variation in temperature gradient along the micro channel, thus the effect of temperature on nanofluid properties are large and using constant properties condition can provide inaccurate results. Also, effectiveness of nanofluid depends on geometry of micro channel and properties of nanofluid, thus, in the present micro channel, effectiveness of nanofluid is negative that means using nanaofluid can’t enhance heat transfer and it is not recommended. Finally, the results of single phase with variable properties and Eulerian-Eulerian two phase model are in a good agreement. Keywords: Numerical investigation, rotating U-shaped Microchannel, hydrophobic surface, aspect ratio, nanofluid, Euler-Lagrange method, Eulerian-Eulerian method, heat transfer, pressure drop, performance coefficient, nanoparticle