Disclosed is a test data analyzing method and system for use in estimation of a defect cause of a product, such as, an integrated circuit, a liquid crystal display, an optical transceiver, a thin film magnetic head, etc., which is fabricated through plural processes. The estimation of a defect cause is achieved by selecting a wafer number to be analyzed, reading test data, reading fabrication line data, counting frequency of machine codes by wafers, grouping test data by machine codes or frequencies, comparing test data distributions between groups by machine codes, and comparing results between machine codes.