Abstract: Based on professor Tygar’s three levels atomicity notion in electronic commerce protocols: money atomicity, goods atomicity and certified delivery atomicity, a new atomicity named contract atomicity （CA） is proposed in this paper, which owns higher level atomicity than that of three atomicity In the CA protocol, electronic contract should be formalized firstly and then be executed integrally A mechanism for executing the electronic contract automatically and another for arbitrating the electronic contract automatically are also proposed As an example, a simple protocol named ACAP（anonymous contract atomic protocol） is designed to illustrate the notion "CA" Through analysis of ACAP using formalized methods, it is proved that ACAP has the following properties: anonymity, security, and atomicity