This paper presents a new DFA methodology to estimate the standard assembly time and evaluate the assemblablity of a product. The assembly time is estimated with some decision and influencing factors of assembly using motion analysis. The assemblability is evaluated using weights of the decision and influencing factors and assembly difficulty scores, and the tables that include the time and score of assembly are suggested. Finally, a new DFA system supporting the DFA is implemented to help designers to analyze the product during the design process in concurrent engineering.