Abstarct: Increasment of heat transfer and energy efficiency due to limited natural resources and reduction the costs of them, always has been one of the most fundamental concern for Engineers and Scientists. This case, particularly in fluid because of small thermal conductivity has great importance. access this, In recent years , one of the main ways to achieve that attention has been paid was adding solid particles with high thermal conductivity at the nanoscale . Researchs in the heat transfer fluid suspensions of solid particles in the nanometer dimensions of the baxse fluid in recent decades has begun . Recent research in nanofluids has been shown that, nanoparticles cause a significant increase in suspension heat transfer.An other ways to improve the thermal performance of conventional devices is usage of mili and micro channels. More fluid has low thermal conductivity rather than solid , so by adding the solid particles the thermal conductivity increase. On the other hand,by adding nano particles the dynamic viscosity of the fluid increases, and followed by have either high power and energy consumption for our pumps. In this survey, flow and heat transfer of nanofluids water - aluminum oxide in micro-tube axial symmetrical laminar flow is numerically studied.Aluminum oxide spherical particles with a diameter of 13, 40, 65, 90, 115 and 130 nm are suspended in liquid water. Nanofluids with volumetric concentrations of 1, 5.2, 4, 5.5, 7, 9% is used to investigation the effect of volume fraction on the thermal conductivity and dynamic viscosity. The relation used for density, viscosity, specific heat and heat transfer coefficient of nanofluids have been considered a function of the volume concentration, particle size and temperature in this study. First, the numerical results with experimental results for friction coefficient and Nusselt number in micro tubes have been validated. With increasing temperature and decreasing volume fraction, nano-fluid dynamic viscosity is reduced, Also with increasing temperature and volume fraction of nano-fluid thermal conductivity is increased compared with the baxse fluid. Increasing the thermal conductivity is higher in low volume fractions. As temperatures rise, lixnks between molecular and thermal capability becomes more, while also viscosity is decrease. For same Reynolds, nanofluid results have been shown that with increasing nano particle volume concentration , the heat transfer and friction coefficient increased. Also under the same pumping power nanofluids heat transfer coefficient higher than the baxse fluid. Finally, two new relations to determine heat transfer coefficient and the friction coefficient for the nanofluids in micro tubes are obtained. In all studies of numerical analysis, software package FLUENT finite volume was used.