Abstract: Based on detail analysis of the chemical composition, mechanical properties and metallographic structure of the J55 casing made by Nippon Steel Corp.,Japan,it is concluded that the major causes responsible for the poor toughness and perforation cracking of the said casing areinappropriately designed chemical composition and coarse gains and poor structure due to inadequate hot-rolling process. Also discussed is the inevitability of the said J55 casing perforation cracking on the basis of result from research concerning perforation safe toughness.