Abstract In this article, flow boiling heat transfer results of ammonia in mini channels are reported. Experiments have been performed to investigate heat transfer in circular vertical mini channels made of stainless steel (AISI 316) with internal diameters of 1.70 mm and 1.224 mm and a uniformly heated length of 245 mm. The test conditions are: mass flux ranging from 100 to 500 kg/m2s, heat flux ranging from 15 to 355 kW/m2 and saturation temperatures of 23 °C, 33 °C and 43 °C. The effects of mass flux, heat flux, vapour quality, saturation temperature and internal diameter on heat transfer coefficients are explored in detail. The local heat transfer coefficients of ammonia with 1.70 mm tube at all vapour qualities and at lower vapour qualities with 1.224 mm tube, are more or less independent of mass flux and vapour quality and are a function of heat flux while the local heat transfer coefficients with 1.224 mm tube at higher vapour qualities are function of mass flux and vapour quality and independent of heat flux. The heat transfer coefficient is observed to be higher for lower internal diameter tube. The heat transfer coefficient is higher for higher saturation temperature at lower vapour qualities and no effect of saturation temperature is observed at higher vapour qualities for both test sections. The experimental data is compared with well known correlations and among them, the Cooper’s  correlation gave best predictions if all data points are included.