A boiling heat transfer test system is built to investigate the boiling flow patterns of the rectangular channels with offset fins, with R134a as working fluid. and the diameter is 1.28mm. As the observation window for visualization research, the upper surface of the test piece is made of transparent organic glass. A video camera is used to shoot the working fluid flow patterns. The experiments are performed at mass flow rate of refrigerant from 68.5 to 305.5 kg/m²s, saturation pressure from 0.27 to 0.45 MPa, heat flux from 9 to 42 kW/m². The results show that the boiling flow in the channels with offset fins mainly presents four kinds of flow patterns: bubbly flow, churn flow, annular flow and slug flow. After the annular flow, with the further increase of dryness, the liquid film finally disappeared, and the boiling flow will enter the dry-out area with liquid refrigerant existed in the form of wet steam. The appearance and end of the four patterns can be influenced by the factors such as mass flow, channel size and heat flux.