Abstract: To prevent the occurrence of low temperature brittle fracture, impact test is generally adopted to check the material toughness, but the cost of impact test occupies a large percentage of manufacture cost of pressure vessel. ASME code gives detailed rules on the conditions under which impact test is mandatory or exempt. However, unclear understanding of concepts and misapplication of ASME code often happens to engineering designers in China. This paper thoroughly sums up the requirements of impact test for carbon steel and low-alloy steel materials and parts in the 2015 edition of ASME code, and provides a flow chart for judging whether a material or component can be exempt from impact test or not so as to help engineering designers to fully understand these specifications.