This study is on the performance characteristics of bubble pump with vertical pipe of different lengths. For the experimental facilities composed of electrical heater used for providing heat source to generate bubble, a vertical tube with Ø4mm diameter, length of 200mm, 300mm, 400mm and 500mm. Storage tank with capacity was of 40 L containing a heat exchanger. The separator and condenser of the air bubble pump were installed separately. The results obtained from this experiment are as follows: there were a few variations of values of temperatures according to vertical pipe lengths of the bubble pump. The mass flow rate was increased by 0.17g/s when it was short as 200㎜ long pipe. But when vertical pipe length was 500㎜, it reached the normal temperature quickly because the slug flow occurred in this case at initial stage. Consequently, it was found that 500㎜ of the vertical pipe length was optimum for best performance of bubble pump.