Abstract: In order to study the heat transfer characteristics of flow boiling in narrow channels, the single-sided steam heating experiments were carried out in a vertical narrow rectangular channel. The influences of mass flow rate, inlet subcooling rate were analyzed, and the prediction equation of flow boiling heat transfer suitable for vertical narrow rectangular channels was established. Currently, most empirical formulas ignore the influence of inlet subcooling on bubble growth process, which results in low accuracy in predicting flow boiling heat transfer. Considering the size limitations of narrow rectangular channels and the influence of fluid surface tension, a new prediction model was established by introducing correlation coefficients. Results show that with the increase of mass flow rate or the decrease of inlet subcooling, the wall temperature increases first, then decreases and tends to be consistent finally. The decrease of the inlet subcooling will greatly improve the heat transfer coefficient, while the mass flow rate has little influence on the heat transfer coefficient. When the inlet subcooling is high, the influence of the mass flow rate is further weakened. The new predicted model could better predict the flow boiling heat transfer coefficient of narrow rectangular channel within the experimental conditions.