Flow boiling, with single phase heat transfer performance in a micro-channel, was investigated in this thesis. Single capillary micro-channel with internal diameter and length of 1.07 and 200mm was simulated by COMSOL Multiphysics software. Inlet sub-cooled temperature of water was 30?C, and three runs including different mass fluxes of 275, 469 and 698 kg/m 2 s was considered. Moreover, the heat fluxes were changed in range of 77 to 131 kW/m 2 . Initially, the single-phase flow within the micro-channel was studied with considering the conservation equations of mass, momentum and energy. When the wall temperature reached up to saturation temperature, nucleate boiling was started and therefore we had two-phase flow in the micro-channel. In this region, two-fluid model was solved including two sets of conservation equations. To related to bubble departure diameter, three different equations was suggested by Tolubinsky, Fritz and Prodanovic. It was found that Prodanovic relation had a high accuracy for predicting bubble departure diameter in micro-channel. Numerical results represented good agreement with experimental results. In next stage, the wall temperature, pressure variation and friction factor of each run were investigated. The results showed pressure drop of two-phase flow was higher than single phase flow, and the friction factor in the two-phase flow is approximately 2.5 times higher than the friction coefficient in the single-phase flow. In flow boiling, the maximum void fraction was near the wall and increased along the channel to 0.003. Moreover, it was observed by increasing the heat flux or decreasing the mass of the flux, the start of the nucleate boiling occurred at a distance less than the entrance of micro-channel and the heat transfer coefficient has also increased. In this study, sub-cooled flow boiling in micro-channel with different bubble departure diameter relation was investigated theoretically which have been few studies in the previous research.