The present discussion will focus on the size effect induced by the variation of dominant factors and phenomena in the flow and heat transfer as the device scale decreases. Due to the larger surface to volume ratio for microchannels and microdevices, factors related to surface effects have more impact to microscale flow and heat transfer. For example, surface friction induced flow compressibility makes the fluid velocity profiles flatter and leads to higher friction factors and Nusselt numbers; surface roughness is likely responsible for the early transition from laminar to turbulent flow and the increased friction factor and Nusselt number; the relative importance of viscous force modifies the correlation between Nu and Ra for natural convection in a microenclosure and, other effects, such as channel surface geometry, surface electrostatic charges, axial heat conduction in the channel wall and measurement errors, could lead to different flow and heat transfer behaviors from that at conventional scales.