The main purpose of this study is to investigate the time-dependent behavior of a sandwich panel numerically and experimentally. The core of the panel is made of poly vinyl carbon Airex 70.75, and shells are made by vacuum injection method from 600 g glass fiber per square meter. To determine the time-dependent compressive and shear modulus of the core, dynamic tests are performed at different temperatures. Using appropriate equations, time-dependent Bulk modulus and Poison's ratio are calculated. Performing three-point-bend tests on sandwich panels, time-dependent behavior of this material is investigated. Finally, these three-point-bend tests are simulated using finite element method and calculated values of viscoelastic parameters. Accordingly, numerical simulation results are compared with experimental data. Regarding the well matching of the quasi-static model, modeling of the relaxation behavior of the beam sample is investigated.