In the digital electronic technology，“logical not”symbols have different representations，and their meanings are not uniform．It leads confusion in studying and application of logic circuits and logic devices．An argumentation is induced for the usage of “logical not”in logical circuit． The correctness of the way and meaning of“logic not”symbol in digital logic circuit are analyzed．The similarities and differences of "logic not in different logic circuits are summarized，and the teaching method of digital logic circuit is strengthened．