In the present study, the characteristics of heat transfer and flow patterns are experimentally investigated on the falling film evaporation of pure refrigerant HCFC123 in a vertical rectangular minichannel consisting of offset strop fins. The refrigerant liquid is supplied to the channel through 37 holes of 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, a transparent vinyl chloride resin plate is placed as the front wall. The experimental parameters are as follows: the mass velocity G = 28-70 kg/(m2s), the heat flux q = 20-50 kW/m2 and the pressure P ≈ 100 kPa. It is clarified that the heat transfer coefficient α depends on G and q in the region of vapor quality x ≥ 0.3 while there is little influence of G and q in the region x ≤ 0.3 . From the direct observation using a high speed video camera and a digital still camera, flow patterns are classified into five typical ones: plane liquid film, wavy liquid film, liquid film accompanied with dry patch, liquid film accompanied with dripping and liquid film accompanied with mist. Then the relation between heat transfer and flow pattern is clarified. The results of heat transfer characteristics are also compared with some previous correlation equations.