3D stacked multi-core processor is one of the applications of 3D integration technology. It achieves high bandwidth access to last level cache and allows to increase the number of cores while maintaining the package area. Although, 3D multi-core temperature increases with the number of stacked dies because of the escalating power density and thermal resistivity. Therefore, 3D multi-cores require lower clock frequencies for keeping the temperature under a safe constraint, so that performance is not always improved. In this paper, we evaluate the performance of 3D stacked multi-cores running under temperature constraints, and we show that there is a trade-off between clock frequency and parallel capability.