Soil compaction is one of the major subjects in a well-mechanized modern agricultural system. In order to reduce the risk of soil compaction, the mechanical impacts to topsoil and subsoil during agricultural machinery operations have to be controlled. Soil compaction test using an off-road vehicle was conducted in the reclamation farmland at Isahaya bay, Nagasaki, Japan. The farmland was formed by sedimentary heavy clayey soil having high water-holding capacity and strong cohesion. In all the tests, the penetration resistance (cone index) of the soil and the bulk density before and after the vehicle running was measured in the field. The detail of soil mechanical properties was obtained from typical soil tests such as the standard compression, oedometer and triaxial compression test, respectively. On the other hand, an unconventional elastoplastic constitutive model for soils having the structure and cohesion was proposed by the superloading-yield/subloading surface model with Hashiguchi and Mase concept (2007). The validity of the model was evaluated by comparisons with the test data mentioned above.