We previously described a parametric eyeball modeling system for real-time simulation of eye surgery (MICCAI 01). However, in the simulation of ophthalmologic surgery, the model of the eyeball alone is not sufficient. The orbital structures are as important as the eyeball. In this paper, we describe the approach to model the orbital structures from patient specific MRI data set and integrate the orbital model with the parametric eyeball model. The orbital tissues including the eyeball, muscles, and orbital fat are segmented from MRI data. An interactive image-based geometrical modeling tool is developed to generate a finite element model of the orbit. Preliminary results include biomechanical models of three human subjects, one of which is a young patient with a benign tumor in the right orbit. The biomedical model can provide quantitative information that is important in diagnosis. It can also be used to accurately analyze the result of intervention, which is an important component of the simulator for training and treatment planning. Our analysis includes a deformation study on an eyeball subjected to simulated tumor growth using the finite element method.