The decomposition of amorphous carbon (a-C:H) films was investigated by thermal desorption under ultra high vacuum. To determine the deposition method dependence of the decomposition, films were prepared by radio-frequency (RF) and electron cyclotron resonance (ECR) plasma chemical vapor deposition (CVD). The a-C:H films were placed in an ultrahigh-vacuum chamber maintained at 10^-7 Pa and heated to 600℃, and the desorption gases were analyzed using a quadrupole mass spectrometer. The thermal desorption spectrum of the a-C:H film deposited by RF-CVD exhibited desorption peaks of Z= 2, 16 and 28 at 400℃, which were assigned to H₂, CH₄ and C₂H₄, and desorption peaks of Z= 16, 28 and 44 at 490℃, assigned to CH₄, C₂H₄ and C₃H₈, respectively. The spectrum for the a-C-H film deposited by ECR-CVD exhibited no peaks for the corresponding mass numbers at 400℃, and only the desorption peak of Z= 44 was exhibited at 490℃, which was assigned to C₃H₈. The a-C:H film deposited by ECR-CVD had higher heat resistance than that synthesized by RF-CVD.