Currently, the significance of a portal stems not only from being a handy way to access data but also from being the means of facilitating the integration with web applications. This paper proposes an interaction instance oriented approach for integrating web applications in portals. The key aspect is to enable a user to have the same interaction experiences from a portal that he/she accesses the web application directly. The approach is thus focused on the description of the presentation layer of the interaction instances of a web application and a portlet, which defines an interaction instance as consecutive web pages or fragments. Web application integration is then transformed to the problem that how to translate all web pages of an interaction instance of a web application to certain view equivalence regions, which form the interaction instance of a portlet. Experiments show that the approach is effective and efficient.