This study aims at measuring the mechanical properties of the colon and developing a constitutive model of them. For this, in vivo compression experiments on the colon of a goat were performed by measuring force-displacement relations and time dependent stress relaxation data under general anesthesia. Force-displacement data were converted to stress-strain data using tissue thickness measured by a laser displacement meter. Compression tests of this study have two phases; a compression phase under three constant compression rates and a stress relaxation phase under constant strain after compression. In this study, viscoelastic parameters were decided at first from the stress relaxation curves, and then nonlinear parameters were decided considering viscoelasticity. In stress relaxation phase, a reduced-stress relaxation function and generalized Maxwell model were applied to express the constitutive model of tissues that have viscoelastic behaviors. Parameters of this model were computed by fitting the constitutive model to the experimental results of stress relaxation. The constitutive model with calculated parameters have shown good agreement with experimental data. Then nonlinear parameters of stress-strain relation were computed. In the compression phase, results revealed highly nonlinear forcedisplacement behavior and strong dependence on compression rates. In this study, Mooney-Rivlin model incorporated convolution integral of viscoelastic constitutive model that has calculated parameters of relaxation phase was applied. Every nonlinear parameter was computed by fitting the constitutive model to the experimental results during compression phase.