Abstract: Grammatical inference（GI） is a problem of learning languages from examples where the target domain is a formal language and the representation class is a family of grammars. A constructive method for inferring context\|free grammars from positive examples is presented in this paper. This work is actuated by the intention to apply GI to replace or supplement the conventional manual grammar construction. The inferring method has the following features: （1） it is interactive; （2） it is incremental; （3） it employs such strategies as stepwise refining and reuse to enhance its learning capability. and （4） the inferred grammars have natural structures.