The aim of the present study are visualization and prediction of passively generated air bubbles through a submerged micro-hole. A simple orifice model, which has been used in most previous studies for bubble formation through an orifice, is modified by considering the friction loss of air flow in a micro-hole with moderate length-to-diameter ratio (4.38 to 13.64). The required driving pressure difference for the generation and detachment of an air bubble can be obtained. In the measurement, liquid was constantly drained out of a test chamber to induce a gradual decrease in chamber air pressure. Air bubble was generated once a threshold pressure difference was reached. The generations of air bubble were observed at liquid drained rates of 0.006, 0.01, 0.02, 0.035, 0.05, and 0.100ml s(-1) through test plates with hole diameters of 1200, 580, 220, 130, 92, and 60 mum, respectively. The predicted volume of a detached bubble and the threshold driving pressure difference for bubble generation and detachment agree well with the measured results. The pressure difference variation in the test chamber can be well predicted by this proposed approach.