The characteristics of heat transfer are investigated experimentally for the vertical falling film evaporation of binary refrigerant mixture HFC 13 4a/HCFC123 in a rectangular minichannels consisting of offset strip fins. The refrigerant liquid is uniformly supplied to the channel through a distributor. The liquid flowing down vertically is heated electrically from the rear wall of the channel and evaporated. To observe the flow patterns during the evaporation process directly, the small circular window is set at the center of every section on the front wall. The experimental parameters are as follows: the mass velocity G =28∼70 kg/(m2s), the heat flux q =30∼50 kW/m2 and the pressure P ≈ 100∼260 kPa. In the case of large mass velocity G ≥ 55 kg/(m2s), the value of heat transfer coefficient becomes lower with increase of mass fraction of low-boiling component HFC 13 4a wb in the region of × ≥ 0.3. The main reason for this inclination of α is considered that shearing force acts on the liquid-vapor interface becomes smaller because of vapor velocity suppressed by higher pressure in the test evaporator in the case of larger mass fraction of low-boiling component. Additionally, mass diffusion resistances formed on each side of vapor and liquid phase along the liquid-vapor interface are considered as a possible cause of reduction in the heat transfer coefficient a with increase of mass fraction wb. In the region of × ≥ 0.8, a descend rapidly despite the difference in the value of wb. It can be attributed to dry-out state of heat transfer area. Heat transfer coefficient derived from experiments is compared with that calculated from empirical correlation equation for heat transfer coefficient of pure refrigerant in a vertical falling film plate-fin evaporator.