The natural convection in acute angle trapezoidal closed cavities is studied where inclined side walls are adiabatic. Unlike closed square cavities, the numerical model of the system shows non-zero flow even at smaller Ra due to the thermal gradient component perpendicular to the gravity. As we increase the Ra value, the magnitude of the flow increases almost linearly. But beyond a particular Ra, sudden changes in flow patterns and heat transfer characteristics are observed, which indicate Rayleigh- Benard type stability breakdown. The transition is similar to that in closed and open cavities. There are few significant differences in the present study from previous researches on the open and closed cavity. The main difference of the current system is that the symmetry breakdown of the system is not associated with the stability breakdown, unlike closed and open cavities.