Provided are methods and systems for monitoring a state of a plasma chamber. In the method, an optical characteristic of plasma generated in a plasma chamber including a window is measured in a predetermined measurement wavelength band. A process status index (PSI) is extracted from the measured optical characteristic. A state of the plasma chamber is evaluated by analyzing the extracted PSI. The optical characteristic of the plasma is measured in the predetermined measurement wavelength band in which a transmittance of light passing through the window is substantially independent of a wavelength of the light.