When driving on a bend, due to the high gravity center and the heavy load, large trucks or lorries are easy to roll-over if their speed is high. This paper proposes a simplified theoretical model of the roll-over process of a truck during its turning, based on the dynamics analysis. Two critical velocities are defined to evaluate the state of the truck: the tilting critical velocity and the roll-over critical velocity. For a truck of given parameters, the main factors that affect the roll-over process are analyzed, including the weight of the truck, the height of the gravity center, and the turning radius of the road. Finally, some qualitative driving suggestions are given about how to avoid roll-over of large trucks.