This paper describes the mechanism of the non-slender wing rock that occurs on a 45° delta wing, by focusing on its unsteady aerodynamic characteristics. In free-to-roll test the wing motions showed irregular oscillations the amplitude of which varied with time. In some cases, the wing suddenly changed its trajectory in the middle of the oscillation, and came to stop at roll angle φ=0°. The static rolling moment is quite nonlinear in characteristics, the profile of which has many critical states. The balance test for constant roll rate motion showed that the amount of time lag was not constant for roll angle, and that it is not linear for roll rate. The results of numerical simulation using the experimental data suggest that it is important to take into account the trajectory of the motion for a better understanding about the unsteady aerodynamic characteristics, associated with the non-slender wing rock.