A new method of monitoring for small leak in the plasma vacuum process using optical emission spectroscopy (OES) system is proposed and compared with other diagnostic tools (pressure gauge, and voltage probe). The result shows that the proposed method (algorithm) is more sensitive to detect gas leakage during plasma discharge operation than other diagnostic tools (pressure gauge, and voltage probe) and also an effective method to detect anomalous discharge (arcing) in the vacuum plasma process. (C) 2010 Published by Elsevier B.V.