This paper is on the formalization of test adequacy criterion for context-free grammars. The basic concept of grammar-based testing is introduced and grammar-based test criterion is formally defined. This paper also proposes a family of grammar-based structural test criteria whose subsume relations and test set sizes are analyzed, which are used to assess the proposed criteria. Some basic properties based on Weyuker?s axiom systems are generalized and some fundamental properties specific to grammar-based testing are proposed. We believe the concepts developed in this paper will contribute to the classical theory of software testing adequacy and are very valuable for software test practice.