Quasi-static and stress relaxation tests were conducted to characterize the rubber material in the Hybrid III dummy neck structure. Test data showed that the rubber can sustain large deformation without failure and the material exhibited nonlinear elastic behavior and high viscous properties. Analytical relaxation responses were obtained for material model Type 6 and Type 77 in the DYNA3D program for linear loading conditions. Based on these analytical responses a numerical procedure was developed for identifying the viscoelastic parameters for the two material types. Due to its generality we found that material Type 77 is better suited to simulate the viscoelastic behavior of the rubber. Using this material model, the relaxation response of the rubber can be captured with a high degree of accuracy.