The thermal stability of amorphous carbon thin films is significantly improved by increasing the carbon-plasma energy in dc magnetron sputtering. The on-set decomposition temperature in air increased from 335°C to 525°C, while the apparent activation energy ΔE in the early decomposition stage increased from approx. 90 to 180 kJ/mol when the pressure in the sputtering chamber changed from 16×10 -3Torr to 1×10 -3Torr. Thermogravimetry (TG) analysis clearly showed that with the increase of carbon-plasma energy, the amount of unbounded and loosely bounded molecules in the structure reduced significantly. A study of decomposition mechanisms based on isothermal TG analysis showed that the decomposition of the films could best be expressed in an apparent nth order reaction, or -[ln(1-α)] n∼t. The apparent reaction order n reduced with the increase of temperature, and also with the carbon-plasma energy during film deposition. The temperature effect on n is explained from the diffusion resistance of decomposed products. The film structure deposited at higher energy carbon plasma showed higher bonding ratio and structure density, which yielded the film of higher thermal stability, ΔE, and lower n values.