Effects of thin film layers have been investigated on the actuating performance of micro heaters. Bubble behaviors on micro heaters were observed experimentally, and heat conduction characteristics in thin film layers were analyzed numerically. Nine (9) kinds of tantalum nitride (TaN) micro heaters were prepared. Step-stress test (SST) showed that maximum endurable voltage levels in the non-passivated heaters are less than 50% of those in the passivated heaters. Open pool bubble test was carried out using de-ionized (DI) water as a working fluid. The non-passivated heaters could produce comparable bubbles with only 20 to 50% of the input energy required for the passivated heaters. However, the non-passivated heaters could operate only in a narrow range of driving voltage. A hybrid model for bubble nucleation was newly proposed to correlate the nucleation times obtained experimentally. The hybrid model adopts different nucleation criterion depending on the input power level. Based on the bubble work obtained from the bubble volume estimated experimentally, actuating efficiencies of micro heaters were calculated and compared. Efficiencies of the non-passivated heaters were much higher than those of the passivated heaters. However, robust actuating characteristics were difficult to obtain in a wide range of power density. Applicability of non-passivated heaters as promising micro actuators needs further investigation from the viewpoints of robustness and reliability.