The modeling of soft tissue behavior is essential in haptic rendering for virtual reality based medical simulators, which provide a safe and objective medium for training medical personnel. This paper presents a measuring device, modeling and effective characterization method of soft tissue's material properties. Experiments were conducted on five porcine livers using an indentation device and ramp-and-hold trajectory. The soft tissue was assumed to be a continuous, incompressible, homogeneous and isotropic solid and a nonlinear constitutive model based on the observation of the force-displacement behavior of soft tissues. The constitutive model was fitted to the experimental data using the Levenberg-Marquardt optimization algorithm combined with a three-dimensional nonlinear finite element simulation. Scripting software was used to automatically characterize the mechanical properties and to repeat the FE simulation until the simulated and experimental responses matched.