We investigated effects of thin film layers on actuating performance of microheaters. Bubble behaviors on microheaters were observed experimentally, and heat conduction characteristics in thin film layers were analyzed numerically. Nine kinds of tantalum nitride (TaN) microheaters were prepared. Step-stress test showed that voltage limits of nonpassivated heaters were <50% of those of passivated heaters. Open pool bubble test was carried out using deionized water as a working fluid. Nonpassivated heaters produced comparable bubbles with only 20–50% of input energy required for passivated heaters. However, nonpassivated heaters could be operated only in a narrow range of driving voltage. We constructed a hybrid model for bubble nucleation prediction correlating nucleation times with driving powers. Based on work of bubble formation estimated from bubble volume evolution, actuation efficiencies of microheaters were calculated and compared. Efficiencies of nonpassivated heaters were much higher than those of passivated heaters. However, nonpassivated heaters failed to show robust actuating characteristics over a wide range of power density. As promising microactuators, nonpassivated heaters need further investigation from a viewpoint of reliability.