Micro heaters for thermal ink jet print heads were investigated to estimate the effects of heater size and operating condition on its thermal efficiency. Tantalum aluminum (TaAl) heaters, 0.15 μm in thickness, were prepared on silicon oxide layer. Silicon nitride film of 0.4 μm thickness covered the heater. A visualization technique was applied to observe bubbles in open pool condition. Volume and lifetime of bubble were measured from CCD images of plane and side views of the bubble generated at the heat flux range of 0.5 ∼ 5.0 GW/m². Thermal conduction simulation for the heater provided bubble nucleation pressure. Bubble work was calculated with Asai's pressure model (Ref. [2]) and the visualization results, bubble lifetime and volume. Then, thermal efficiency of the heater defined as the ratio of bubble work to the electric input energy could be estimated.