The assumption of flow continuum can usually be accepted for flow in MEMS with channels from tens of micrometer (Kn~0.001) to fractions of 1 millimeter (Kn~0.0001). Hence, the mechanism for the departure of flow and heat transfer correlations for MEMS from standard ones can be largely attributed to the variation of dominant factors in flow and heat transfer when the scale goes down. Both analytical and experimental results show that the compressibility for the gas flow in the micro channels can be very remarkable. And it leads to the increase of friction factor and the Nusselt number. The surface roughness should be another factor which is responsible for the different flow and heat transfer behavior in MEMS. Experimental results show that the flow behavior in the micro channels for the smooth tubes is very close to usual one, but changes remarkably even when the relative roughness is 2-4 % only. As the object size going down, the dominant forces will vary, for example, the viscous force is dominant over the inertial force for natural convection at small Grashof number, and consequently, the balance between the buoyancy and viscous force leads to a heat transfer correlation, Nu~Gr1/3, which differs from standard one.