Reworking in developing software exists widely in IT project management. But the current estimating methods on project duration cannot deal well with this problem. In this paper, some key concepts, such as adjusted activity, adjusted GERT network, expected project duration, expected critical path, are firstly defined. Then authors study estimating method on project duration for different rework modes, including serial, paratactic, and combined structure by encapsulating of activities. Finally, a general estimating method on IT project duration oriented to GERT is presented, and detailed steps are illustrated with an example project on IT software development.