Bilateral contracts are commonly existed during the social economic activities.Considering the benefits of both parties on the balance of debt,the defense right of simultaneous performance is prescribed in the 66th article of Contract Law of People's Republic of China.Due to the generality and ambiguity of the regulation itself,a lot of puzzles occurred when the regulation put into the practice.This paper aimed to make the positive research on the causes,elements,restrictions and legal effect for the regula...