New applications of HFC refrigerants in organic Rankine cycles at high saturation temperatures and the wider use of CO 2 for air-conditioning have pushed research to the characterization of two-phase heat transfer at medium/high reduced pressures and have pointed out the effect of these operating conditions on asymmetric distribution of refrigerant around tube perimeter and its indirect effect on heat transfer. Currently there is a lack of data about asymmetric distribution of liquid film at the wall, especially for refrigerants and micro-channels. In order to have a physical evidence of this asymmetry also for micro-channels and approach to a relationship between this phenomenon and dimensionless parameters, new data are here presented. The asymmetric annular flow of the refrigerant R245fa inside a horizontal, round 2.95 mm inner diameter channel is studied with pictures captured by a high speed video camera. The experimental results here presented were obtained at saturation temperatures equal to 20 °C and 40 °C at low mass velocities (50, 100 and 200 kg m -2 s -1 ) to asymmetric distribution, enriching the database presented in previous studies. The new dimensionless parameter, eccentricity, has been related to the dimensionless groups: Froude and Bond numbers, and Martinelli parameter, showing the mutual correlation among them.