Internet, to locate target services in an accurate and efficient way becomes increasingly difficult. At present, semantic web service is regarded as the most promising approach to address the challenge. In this paper, an ontology service description language and a query language, OSDL and OSQL are proposed respectively, and then a service matchmaking algorithm based on interface semantics is proposed. The problem of service matchmaking can be attributed to the similarity between the semantic pair, and the similarity is the quantization of their relationship, which is discovered through reasoning on ontology. Unlike the existing semantics-based service matchmaking algorithms, which focus on the relationships among the successors and ancestors of ontology classes, the proposed one also pays attention to the comparability among classes and their property classes. The experiment data indicate that it is worthwhile to take the point into consideration to receive a higher recall and precision.