In present investigation, numerical simulations are performed to study flow and thermal field of non Newtonian fluids in rectangular microchannels. The flow is considered to be slip, three dimension, steady, incompressible, laminar, and power law model is used to characterize the behavior of the non-Newtonian fluid. The constant wall heat flux both axially and circumferentially and constant wall temperature are employed as thermal boundary conditions. The uniform velocity and temperature profiles are considered at the entrance of microchannel. The governing equations, continuity, momentum and energy, are non-dimensionalized with respect to specific variables. The set of governing differential equations with appropriate boundary conditions are solved together using control volume finite difference method. Axial velocity profiles, centerline velocity , the product of friction factor and Reynolds number, dimensionless bulk temperature, the local Nusselt number, dimensionless wall temperature for constant wall heat flux and dimensionless wall heat flux for constant wall temperature boundary conditions for a wide range of power law indices(n), Reynolds number (Re), Prandtl number (Pr) and slip coefficient(?) are obtained. Results show the product of friction factor and Reynolds number and centerline velocity increase with increasing power law index while considering wall slip results in reduction of those parameters. Relative difference of the product of friction factor and Reynolds number between slip and noslip conditions enhances with increasing power law index. consequently slip effect in dilatants fluids are stronger than that of pseudo plastic fluids. Increasing Re, Pr and ? enhances the local Nusselt number for both thermal boundary conditions while increasing power law index reduces the heat transfer coefficient. Increasing Re, Pr, ? and n has significant effects on dimensionless bulk temperature for both thermal boundary conditions. Relative difference of the Nusselt number between slip and noslip conditions increases for higher power law indices, consequently slip effect in dilatants fluids are more than that of pseudo plastic fluids. In case of constant wall heat flux, relative difference of the local Nusselt number between slip and noslip conditions is more than that of in constant wall temperature for similar power law index, therefore slip effect in constant wall heat flux is more than that of constant wall temperature boundary conditions. Considering the viscous dissipation effect cause decrease in Nusselt number in both thermal boundary conditions and the viscous dissipation effect in dilatants fluids ...