Abstract: Information Service is still a hot topic for information sharing research. This paper presented the concept and mathematics model of semantic relatedness based information focusing. Information focusing, as a new information service,implements information congregating by using the semantic association relationship between information units.This paper presented the Ontology-based metadata model,and gave mathematics model of information and information space, and presented the mathematics about information focusing. By giving the formally definition of semantic relatedness,this paper also defined the concept of information focusing based semantic relatedness.

Key words: Information space, Information focusing, Ontology, Semantic relatedness