In this paper, we investigate how the methods developed for using in Semantic Web technology could be used in validating requirements specifications. The goal of our investigation is to do some (at least partial) checking and validation of the UML model using a predefined domain-specific ontology in OWL, and to process some checking by using the assertions in descriptive logic. We argue that the feedback caused by the UML model checked by ontologies and OWL DL reasoning has an important impact on the quality of the outgoing requirements. This paper describes not only methods but also the implementation of our tool TESSI (in Prot´eg´e, Pellet, Jess) and practical experiments in consistency checking of requirements.