HFO-1234yf has similar thermodynamic properties to HFC-134a but much lower GWP value. It is expected as a good candidate to replace the refrigerant HFC-134a in the near future. However, only very few papers have been published in the past years regarding to the flow boiling heat transfer performance of this new refrigerant, but some of their results are still not in consistent with each other. This study provides an experimental measurement of flow boiling heat transfer and pressure drops of refrigerants HFO-1234yf and HFC-134a in a small circular tube. The test results show that both pressure drop and flow boiling heat transfer performance depend on the fluid properties, flow conditions and flow patterns. The major controlling properties on pressure drops and heat transfer coefficients of flow boiling is strongly depending on their two-phase flow pattern at various flow conditions. The flow pattern analysis is able to explain the inconsistency between the test results in the previous published literatures on flow boiling heat transfer and pressure of refrigerants HFC-134a and HFO-1234yf adequately. (C) 2018 Elsevier Ltd. All rights reserved.