ALD Al2O3 thin films were accomplished by electron cyclotron resonance (ECR) plasma-enhanced chemical vapor deposition using trimethyl aluminum (TMA) and O2 as precursor and oxidant, where argon was kept flowing in whole deposition process as a purge and discharge gas. The more than 3.2 Å/ purging cycle deposition rate and 8.3 dielectric constant of deposited Al2O3 was obtained even without extra heating, which were characterized including X-ray photoelectric spectroscopy (XPS), atomic force microscopy (AFM) and high resolution transmission electron microscopy (HRTEM). Deposition rates and surface morphology affected by RF bias potential were emphasized in in this paper