Due to the presence of interfaces, heat transfer can be enhanced by the vortices in liquid plugs in microchannel heat exchangers. The heat transfer in liquid plugs moving in microcapillaries with constant-surface-heat flux boundary condition is investigated. The e ects of the Peclet number and the plug length are studied. Higher Peclet numbers and shorter plug length result in higher Nusselt numbers and lower maximum uid temperature. However, higher Peclet numbers require higher ow speed, while shorter plug lengths result in higher flow resistance coefficients. The pressure drop needs to be considered in the optimization of microchannel heat exchangers.