Abstract: With Sharpy impact specimen,an experimental study was carried out on the dynamic fracture toughness K_Id of heavy rail steel using a pendulum impact tester. The dynamic geometrical factor f(a/W) for heavy rail steel was measured with the method of dimensionless flexibility. The test result has shown that the loading rate and experimental temperature including the range of specimen's width have a considerable effect on K_Id. The present achievement has a certain referen-tial value for the evaluation of dynamic fracture toughness of heavy rail steel produced by Panzhi-hua Iron and steel Corp.