Along with advancement of technology, scientists had built the air conditioner device whereas in the working system, there are fluid used as heat delivers known as refrigeran. The R134a refrigerant or Tetrafluoroethane (CF3CH2F) were used in this research. The refrigerant included to HFC (Hydro Fluoro Carbon) had no Cl (Chlor) element, colorless substance, odorless and had boiling point at room temperatur although it still had high enough of GWP. Based of system, the type of refrigeration system often found were vapor compression refrigeration system which the mixed phase changed to vapor phase and then it called evaporation process. One of the parameter of evaporation process’s calculation were the mass flow rate of refrigerant and the value of vapor quality which had an effect on the value of coefficient heat transfer evaporation of R134a refrigerant. The research used a simply modified vapor compression refrigeration system with added orifice tools, U-manometer, heater, and test section were the main tools. The orifice had 2 mm diameter hole with a U-manometer containing Mercury, Heater uses a nickel wire were heated with a voltage regulator, while the test section is a double pipe type which on the outer pipe has 27.5 mm wall diameter and the inner tube has 16.6 mm wall in diameter with 1 mm thickness of the pipe. Variations were performed on the inverter with 14, 16, 18, 20 and 22 Hz frequencies to obtain the value of mass flow rate of refrigerant. Voltage variation on voltage regulator used to know the vapor quality of refrigerant with 20 – 54 volts range. Some literatures had described flow patterns that occur, one of them was based on the position of the channel, then the position of the vertical pipe channel was used as one of the variations of this research. The result of experimental research on variation of mass flow rate of refrigerant known as mass flux of refrigerant with the alteration of refrigerant vapor quality which happened on smooth channel of vertical pipe with downward of flow doesn’t show clearly increase the value of coefficient heat transfer evaporation. The value mass flux of refrigerant variation varies between 34.58 kg / m2.s - 49.16 kg / m2.s while the value of coefficient heat transfer evaporation ranges from 1024.72 W / m2.K - 2366.96 W / m2.K with value of vapor quality between 0.162 - 0.96.