Abstract：The traditional contract theory emerged in 19c when Laissez-fair policy and the principle of autonomy of will were widespread.At that time, contract is a consensus.However, as governments paid more attention to social welfare and the fairness of contract in 20c, the complete freedom of contract was restricted.As a result, a special type of contract named monopoly agreement was initially regulated by laws, which was a breakthrough of traditional contract theory.Based on the legal obligation rather than the contractual obligation, it emphasizes the element of effect except for normal requirements.

Keyword：contract;monopoly agreements;legal obligation;constitutive requirements;