Lifetime and failure mode of the micro heater within thermal bubble inkjet is investigated in this study. Operational lifetime is defined as the firing times for electric pulse signal continuously supplied to micro-heaters until thermal bubble failed to generate on heater surface. Input pulse signal and thickness of the thin-film within micro-heater are two of the most important factors, which may affect the inkjet operational lifetime. In the study of input pulse signal, operational lifetime is significantly affected by the power density, which is defined as the heating power divided by the micro heater surface area. The measurement tricks also cause the significant difference in heater lifetime measurement. Heater lifetime of close-pool trick is about one order higher than that of open-pool trick, which is caused by the extra collapsing force on the thermal bubble. In addition, the failure mode of micro-heater is also observed by using optical microscopy in this study. Phenomena of crater cracks, broad swell and black points on the heater surface are observed which caused the failure mode of micro-heaters. It is noted that the operational lifetime is not obviously influenced by different thin film thickness, which includes layers of TaAl, SiC and Si₃N₄, design within 1.3 micrometer in this study.