A fine-grained behavior model matching based method and system for effective feature location in large legacy systems. A feature behavior analyzer extracts internal behaviors of a feature under requesting based on NLP techniques or writing rules of the feature specification and generates a feature behavior model. A method uses multiple information sources associated with each method under analyzing to generate an expressive behavior signature for each method. The method integrates control flow information of each method and the signature information of its callees, and generates a CFG-based behavior model. A feature location identifier identifies and ranks the feature-related code locations based on a similarity between the feature behavior and the code behavior models. In one aspect, “use cases”, “source code repository” and “issue tracking system” are historical information of existing applications that are used to help understand each code unit of legacy systems applications, and recommend code units related with the given feature description.