A method for evaluating reliance level of a virtual metrology system (VMS) is proposed. This method calculates a reliance index (RI) value between 0 and 1 by analyzing the process data of production equipment to decide if the virtual metrology result is reliable. A RI threshold is also defined in this method. If a RI value is higher than the threshold, the conjecture result is reliant; otherwise, the conjecture result needs to be further examined. In addition to the RI, the method also proposes process data similarity indexes (SIs). The SIs are defined to evaluate the degree of similarity between the input set of process data and those historical sets of process data used to establish the conjecture model. Two kinds of SIs are included in the method: global similarity index (GSI) and individual similarity index (ISI). Both the GSI and ISI are applied to assist the RI in gauging the reliance level and locating the key parameter(s) that cause major deviation, hence the VMS manufacturability problem is resolved. An illustrative example with 300-mm semiconductor foundry production equipment in Taiwan is demonstrated in this work. The real experimental results show that this method is applicable to the VMS of (such as semiconductor and TFT-LCD) production equipment.