This paper proposes an algorithm dedicated to estimate on-line the risk of rollover of an All-Terrain Vehicle (ATV), together with the maximal admissible speed preserving its stability. This work considers the case of off-road motion, subjected to low and varying grip conditions. As a result, an observer using several layers is developed to account for sliding. This permits to feed a dynamical model of the ATV, which is on-line adapted and consequently relevant. This model is exploited in two ways. First, it permits to estimate on-line the Lateral Load Transfer (LLT), representative of the level of stability. Secondly, the model is used in order to derive the velocity leading to a risk of rollover. Vehicle actual velocity is then limited to this value, considered as the maximal admissible speed, so that stability can be ensured anytime.