A new optical monitoring method for thin-film coating is described. Based on the admittance loci theory of Macleod, the traditional turning point method, and level monitoring method, the new method is able to compensate the coating error and break the transfer of error to the subsequent layers. The method can be applied to monitor deposition of quarter-wave optical thickness films or nonquarter-wave optical thickness films. Theoretical analysis shows the method can effectively improve the monitoring accuracy. © 2004 Society of Photo-Optical Instrumentation Engineers.