Mode Generation method such as Dynamic and Static Correction Mode used in RecurDyn has some limitation. Total number of nodes and element in one body is limited because of working memory problem used in mode generation codes and cannot consider special boundary effect such as various constraint equation. So RecurDyn/RFlex supplies another method to generate attachment and constraint modes. In this method, user define special boundary or applied force and FE software generate displacement vector. With result date written from FE software, RecurDyn/RFlex convert these data as a mode shape and add to original RecurDyn/RFlex input file.