Nano fluids are a new division for heat transfer enhancement. But Nano fluid Flows and heat transfer analyses are more complicated than their base fluids. Nonlinear variation of thermo physical properties of Nano fluids with variation of temperature and volume fraction of nanoparticle, the shape and size effects on properties and effective mechanism in thermal properties of Nano fluids must be concern in analyses. The Reynolds number in Nano fluid studies depends on the inlet and the kinematic viscosity of Nano fluid. The Nano fluid kinematic viscosity increases with an increasing in nanoparticles volume fraction while the inlet velocity should be increased to keep the Reynolds number constant. Therefore, it is not clear that either increasing the nanoparticles volume fraction or increasing the inlet velocity has major role on heat transfer enhancement in Nano fluids flow studies which are done at constant Reynolds numbers, In this study, forced convection AL 2 O 3 - Water Nano fluid laminar flows in two dimensional rectangular micro channels have been investigated to study thermal efficiency and heat transfer enhancement due to addition of the nanoparticles to the base fluid and analyses the effects and sensitivity of variational proprieties of Nano fluid with temperature especially in micro channels at low Reynolds number. Two different cases are examined to evaluate proportion impact of increasing nanoparticles volume fraction (?) and the inlet velocity ( ) on the heat transfer enhancement and in third case the slip velocity and temperature jump with constant temperature condition at the wall for a Nano channel have been investigated. Two dimensional Navier stokes and energy equations accompany with the slip velocity and the jump temperature boundary conditions expression have been discretized using the central finite difference method. The constant and variational properties of Nano fluid have been considered to determine the thermal conductivity and kinematic viscosity of Nano fluids. The calculated results show good agreement with the previous numerical and analytical data. It is found that at a given Reynolds number, the major enhancement in thermal efficiency is not due to increasing the Nano particle concentrations but it is due to the increasing the inlet velocity to reach a constant Reynolds number. Constant Reynolds number studies of Nano fluids are not sufficient approach to evaluate the heat transfer and the skin friction factor due to nanoparticle usage. The Nuselt number in thermal studies depends on the heat transfer conductivity coefficient and the heat transfer convective coefficient. Hence the heat transfer convective coefficient is depends on the fluid and flows properties. The result shows heat transfer enhancement hence the Nuselt number is decrease in variational properties Nano fluid analyses and it is because of the velocity profile change in Nano fluid analyses with variational properties. So Nuselt number studies of Nano fluids with variational properties are not sufficient approach to evaluate the heat transfer and the skin friction factor due to nanoparticle usage. Result shows good enhancement in nano fluids heat transfer efficiency in front of their base fluid with using little volume fraction of nanoparticle but this Thermal Efficiency decrease with increasing volume fraction of nanoparticle. Keywords: Nano fluid, Micro channel, Thermal Efficiency, Vibrational Properties