Viscous dissipation effects in circular micro-tubes and rectangular micro-channels were investigated theoretically in a fully developed laminar flow region. From the energy conservation law, simple theoretical equations were derived for the evaluation of the viscous dissipation effect. The proposed equations were verified by comparing to other researchers’ experimental measurement and numerical results. Wall heat flux, fluid velocity, operating temperature, tube diameter, and working fluids’ effects on viscous dissipation were evaluated. It was found that viscous heating increases with decreasing aspect ratio in rectangular channels. The classical macro methods for heat transfer and pressure drop predictions agree well with the measurement and numerical predictions in micro-channels. Finally, a criterion for the evaluation of viscous dissipation was provided with the Brinkman number and heat balance concept.