Abstarct: The boiling heat transfer, especially the subcooled flow boiling, is one of the cooling systems being used in industries due to their high heat transfer coefficient and low wall super-heat. The subcooled flow boiling happens when the bulk flow temperature and the interface temperature are lower and higher, respectively than the saturated temperature corresponding to the flow pressure. In the current study, an experimental apparatus was constructed, and subcooled flow boiling in an annulus tube was investigated. The annulus tube is in the vertical direction, and the internal and external diameters are 50.7 and 70.6 mm. The operating pressure was 1 atm, and the working fluid was water. In this investigation, heat flux and mass flow rate effectiveness on heat transfer coefficient are considered. The experiments were performed in the mass flow rate range of 700 ml/min to 1700 ml/min and heat flux 5kw⁄m^2 to 30kw⁄m^2 with subcooling 50℃,30℃ . In order to validate the results of the experiment, the most commonly used equations for predicting heat transfer coefficient single phase and subcooled boiling in annulus were used and it was found that these relationships can accurately predict the results of single phase and subcooled boiling experiments. The results show that the heat transfer coefficient is highly dependent on heat flux in a direct relationship and by increasing heat flux will be increased So that in the heat flux 5kw⁄m^2 to 30kw⁄m^2 , the heat transfer coefficient varies between ( 223W)⁄(m^2 k) to ( 713W)⁄(m^2 k). Also, the mass flow reduction causes heat transfer coefficient increment in subcooled boiling regions. It was also found from the subcooling change that the decrease in subcooling increases the heat transfer coefficient which has the greatest contribution to increase the heat transfer coefficien. In this experimental study, the setup was designed to allow the observation of infrared subcooled boiling phenomena from outside the setup and to measure the heat transfer coefficient at two specified elevations of the vertical tube.