A method and a system for estimating an assembling-related fraction defective coefficient of an article in the stage preceding to manufacturing, e.g. at a stage of design. Assembling operation, properties/conditions of parts to be assembled and conditions of an assembling shop having significant influence to the likelihood of occurrence of failure in assembling work are inputted as data. Estimated value of assembling-related fraction defective is arithmetically determined with high accuracy by executing an assembling-related fraction defective value estimating program on the basis of the data as inputted.