The difference of strain energy is concentrated on joint area between beam and shell modeling of box channel structures. Therefore, the analysis errors from the beam modeling are basically due to this strain energy difference. At concept design stage of automotive body we usually use the beam model. However, this kind of error at joint area is so dominant on the analysis that a correct beam modeling technique is necessary.There are some conventional beam modeling techniques in this problem. The most widely used one is using spring elements at the joint area. But the spring element has some problems in application to sensitivity analysis because it is a zero dimension element which does not have material properties and section properties such as mass, area, etc.This study has been done to suggest a beam modeling technique of box channel structures using modified section properties of joint area, and also to introduce some important nondimensional factors that govern the analsis errors on those by numerical experiments. The modified beam modeling suggested in this study has showed the same results as the shell modeling in both static and normal mode analysis. This study has also showed the minimum model length of structure to extract the pure joint effects and to remove the boundary effects. And also this study has showed that the most dominant factor in beam modeling of box channel structures with joint is the ratio of thickness and section length, t/H.