A suction line of a refrigeration cycle with vertical upward flow is one area where the refrigeration oil tends to accumulate. This paper discusses the flow characteristics of the oil film in a pipe with the vertical upward flow. Air and oil two-phase flow is used as the first step of the study and the flow pattern of the oil film in the vertical upward flow is observed. The oil film thickness is measured with changing flow rate of both the air and oil, viscosity of the oil, diameter of the pipe and pressure in the pipe. The condition under which the oil film having a certain thickness can flow upward is proposed as a design criterion for the refrigeration cycle. A general expression with non-dimensional parameters is developed based on the criterion. In addition, measurement of the oil film thickness in a refrigeration cycle by using a capacitance sensor is introduced and the characteristic of the sensor is clarified.