Flow visualization of gas-liquid two-phase flows in circular glass tubes with inner diameter of 500 and 300 孠have been conducted. The 300 孠tube is further utilized for the investigation of the heat transfer characteristics under constant heat flux at wall. The constant heat flux is supplied by joule heating via applied electric current through a thin uniform indium tin oxide coating. To ensure steady flow patterns, the two-phase flow is achieved by injecting nitrogen gas coaxially through a centrally positioned tube to the continuous liquid phase flow. Measurements of wall temperature along the heating zone, various flow patterns and pressure drop are recorded simultaneously. Thermal performance is found to depend on bubble size, void fraction, Reynolds number and flow patterns. The two-phase Nusselt number (NuG,L) for bubbly flow is found to increase by 176% compared to the single phase flow, while the corresponding pressure drop increases by less than 27%.