The flow calorimeter is a device used to measure the calorific value of gas fuel. Each fuel certainly has a different calorific value. The heat transfe will affect the performance of the device and further more affect the efficiency of device. In this research used LPG (Liquified Petroleum Gas) as fuel and water as heat absorber. The variables used are variations of valve opening gas ⅛, ¼ and ½ with the water discharge of 2 LPM. The experiments were conducted by the time interval of 60 minutes from each variation. The result obtained in this research showed that the valve opening gas affected the increasing of calorific value and efficiency of flow calorimeter.