The upward flow boiling experiments were carried out in a flat aluminum extruded multiport tube, which is composed of 7 parallel rectangular channels (1.1 mm × 2.1 mm) with hydraulic diameter of 1.4 mm. Two refrigerants, R245fa and R1234ze, the latter a recent environmentally safe refrigerant proposed as a potential replacement for R134a, were tested. A new hot water heating technique that accounts for either uniform or non-uniform local heat flux distribution along the channel was developed to obtain and reduce the data. Effects of heat flux, mass flux, vapor quality, and saturation temperature on flow boiling heat transfer in multiport tubes were considered. Finally, the experimental results were compared with some well-known correlations to evaluate the capabilities of existing prediction methods. The analysis shows that the three-zone model for slug flows works well for that subset of test results, utilizing the apparent surface roughness in place of the dryout thickness in the model, as has been previously done for silicon, stainless steel and copper microchannels with measured surface roughnesses.