A method for evaluating reliance level of a virtual metrology system is disclosed. In this method, a reliance index (RI) and a RI threshold value are calculated by analyzing the process data of production equipment, thereby determining if the virtual metrology result is reliable. Besides, in this method, a global similarity index (GSI) and individual similarity indexes (ISI) are also provided for defining the degree of similarity between the current set of process data and all of the sets of historical process data used for establishing the conjecture model, thereby assisting the RI in gauging the degree of reliance and locating the key parameter(s) that cause major deviation.