A pipes inspection robot prototype was built and a mathematical modeling of its dynamics was developed. In order to pass over complicated pipeline networks, the robot was constructed with a flexible rubber structure moved by pneumatics power The robot locomotion was inspired by the inch-worm gait kinematics. Because of significant acceleration values revealed during the robot gait, the robot dynamics was mathematically formulated either by a single degree of freedom model or by the assumed mode summation method. A set of experiments were conducted for obtaining all the parameters required for models formulation. Finally the models were validated by comparing the numerical and experimental robot gait with time.