Dynamic behaviour of a thin-walled beam due to impact are investigated through experiments, and a particular attention is paid to lateral buckling of the beam : T-shaped beam is chosen for tests and impact force is applied by a pendulum-type impact testing machine, of which weight itself is a load-cell and the impact force is measured. Strains of the beam are also measured by strain gages. On the other hand, static tests for lateral buckling of beam are carried out to examine the dynamic effects on the ultimate strength. These experimental results are compared with the calculated ones by the finite element method. The dynamic behaviours of the beam-component of frame structures are clarified through the investigation.