In this paper, the relations between ice resistance, shipspeed and ship motions are investigated with a numericalmodel simulating the icebreaking process with ship'smotions in 6 degrees of freedom (DOF). The coupledmotions of ship are considered in all 6 DOF togetherwith the corresponding environmental forces. The couplingbetween ship's movement and the breaking of iceis solved step by step with the update of ice edge geometryand ship's kinematics variables. Time series ofglobal ice loads (forces and moments) and ship motionsare obtained correspondingly. The effect of ship's verticalmotions is studied by comparing results betweenreduced-order-models and the 6-DOF model with hullspeeds and ice thickness being parameters. The numericalmodel was validated with the Swedish icebreakerTor Viking II in previous works, and this vessel is usedin this paper too as the physical prototype of the casestudy. Results are compared to full scale data as well asLindqvist’s empirical formula.