The purpose of this study is to grasp running characteristics of wheel system vehicle running on soft terrain. The influence of tire inflation pressure on running characteristics of tire is investigated based on a laboratory experiment and a proposed simulation. The object of the test is 1000-R20 off- the- road tire with initial inflation pressure of 550kPa. It was observed that the sinkage is accompanied by the set of the inflation pressure of the tire. The result shows that running performance of vehicle could be controlled by setting of inflation pressure. Furthermore, it is revealed that tractive force increases with decrement of inflation pressure.