Alumina washcoated cordierite monoliths are prepared to investigate mechanical stability of monolithic catalysts by ultrasonic vibration and thermal shock. It is shown that mechanical failure of monolithic catalysts is a stress-induced fatigue damage process, i.e., microcracks are first initiated at the pre-existing flaws and then propagate to form macrocracks, leading to the washcoat surface cracking, interconnection between large cracks, and consequent spalling of active material. It is also found that the washcoat adhesion of mini-monolithic catalysts is distributed over a wide range of values, resulting from fatigue failure of active material. Statistics indicates that the measured weight losses follow a bimodal Weibull distribution, attributed to the transition in the type of the flaws that control fatigue, i.e., the transition in the two sinning modes, interior spalling (cohesive failure) and interface spalling (adhesive failure). Furthermore, it is recommended that at least five monolithic specimens be used to obtain a reliable mechanical stability of monolithic catalysts.