This paper presents a methodology to automate natural language requirements analysis and class model generation based on the Rational Unified Process (RUP). Use-case language schemas are proposed to reduce complexity and vagueness of natural language. Some rules are identified and used to automate class model generation from use-case specifications. A CASE tool named Use-Case driven Development Assistant (UCDA) is implemented to support the methodology. UCDA can assist the developer to generate use-case diagrams, use-case specifications, robustness diagrams, collaboration diagrams and class diagrams in IBM Rational Rose. It helps accelerate requirements analysis and class modeling, and reduce the time to market in software development.