There are several purposes for modeling and analyzing the problem domain before starting the software requirements analysis. First, it focuses on the problem domain, so that the domain users could be involved easily. Secondly, a comprehensive description on the problem domain will advantage getting a comprehensive software requirements model. This paper proposes an ontology-based approach for mod-eling the problem domain. It interacts with the domain users by using terminology that they can under-stand and guides them to provide the relevant information. A multiple paradigm analysis approach, with the basis of the description on the problem domain, has also been presented. Three criteria, i.e. the ra-tionality of organization structure, the achievability of organization goals, and the feasibility of organiza-tion process, have been proposed. The results of the analysis could be used as feedbacks for guiding the domain users to provide further information on the problem domain. And those models on the problem domain could be a kind of document for the pre-requirements analysis phase. They also will be the basis for further software requirements modeling.