A gas-liquid stratified flow boiling was performed in a 1530 mm long copper tube with an inner diameter of 12 mm under forced convection condition. Four different heat fluxes (q = 15.43 kW/m(2), 18.52 kW/m(2), 21.60 kW/m(2) and 24.69 kW/m(2)) and inlet subcooled temperatures (Delta T-sub =10 degrees C, 15 degrees C, 20 degrees C and 25 degrees C) were applied to investigate the characteristics of flow boiling instabilities, respectively. The results show that the instability features were closely related to the heat flux and inlet subcooling degree. With the increase of heat flux or decrease of inlet subcooling degree, two types of instability oscillation, DWO (density wave oscillation) and PDO (pressure drop oscillation) were identified. Moreover, an additional enhancement of heat transfer coefficient was found when the transition of the oscillation type from DWO to PDO happened. In the meantime, the frequency and amplitude of the PDO type oscillation were also increased with the rise of heat flux or the drop of subcooling. The intensification of fluctuation in PDO, however, has deteriorated the flow boiling heat transfer. (C) 2017 Elsevier Ltd. All rights reserved.