Dynamic buckling behaviour of a T -section beam subjected to impact loads is consecutively investigated. In the previous reports it is found that even in the case of higher impact loads the lateral buckling did not always occure. In this report conditions of occurrence of the buckling is investigated by series tests. Results of experiments show that the momentum of a impact weight is a main factor for the occurrence of the buckling. The occurrence of the buckling depends on the momentum. This result is discussed with a simple analysis.