The effect of parallel flow of water on the frequency of bubble formation from a single-hole nozzle placed in a vertical pipe is investigated under terrestrial gravity (1-g) and micro-gravity (mu-g) conditions. The parallel flow means that the water now in the pipe is parallel to the air flow emitted from the nozzle. The micro-gravity conditions were realized using the drop tower of the Japan Microgravity Center (JAMIC). A force balance equation based on a one-stage model was introduced to predict the frequency of bubble formation under the two gravity conditions. The predicted value could approximate the measured value regardless of the gravity level when the frequency of bubble formation,f(B)(-), was relatively low. When the one-stage model was valid, the buoyancy force, surface tension force and drag force acting on the bubble just before detachment from the nozzle tip almost balanced under the terrestrial gravity (1-g) conditions, while the latter two forces balanced under micro-gravity (mu-g) conditions.