Current web based technologies are not good and accurate for the purpose of finding correct data from the web resources. They also failed to extract the required data. Data representation, data interpretation and data maintenance are the main issues of current web technologies. These problems can be reduced by using semantic web vision by giving contents well-defined meaning via ontologies to enable them machine-understandable. Web portals provide forums to read, write and exchange change data over the web to a group of people having shared interests. The existing web portals have been developed using current web technologies. As a result there are a number of issues for data interchange between users. To overcome these limitations, Semantic Web technologies have been developed and being used. In this paper, a new approach based on formal methods to develop web portals has been proposed. Formal methods are the techniques based on mathematical logic and have been used for the specification, design and analysis of software systems. WSDM, Web Semantic Design Method has been used as a basis for this paper. The core components such a Mission Statement, Audience Classification, Audience characterization, Task Modeling and Information Modeling has been formalized in Z, a formal specification and modeling language. The resulting models have been verified by using Z/EVES theorem prover. Keywords: Semantic web, WSDM, Ontology, Formal Methods, Z notation, Z/EVES, Verification.