As a cask material, ductile cast iron may be susceptible to failure in a brittle manner under certain temperature and load conditions. A design criterion for ductile cast iron casks against brittle failure due to drop tests, has been proposed by Central Research Institute of Electric Power Industries. This design criterion includes a safety factor which presents the extent between the detectable flaw size and the critical flaw size and may be interpreted as ``uncertainty factor`` as to the uncertainties regarding stress prediction, fracture toughness and so on. In this report, to verify the proposed design criterion, probabilistic evaluation was performed according to a series of drop tests using a full scale cask and material tests, and it is confirmed that the proposed design criterion is applicable and reliable. Furthermore, applicability of the safety design method described in the IAEA-TECDOC-717 published in August 1993 was investigated. (author).