Sub-threshold designs play an important role in energyconstrained applications. In those designs, path delays depend exponentially on threshold voltage/temperature. As such, dynamic configurations at runtime are desired for best trade-off between operating power and performance. Unfortunately, most existing works only consider either process or temperature variations but not both, resulting in sub-optimal configurations or even functional failures. Moreover, little study has been performed on the graceful degradation of sub-threshold designs, which is important in the presence of drastic delay variations. Towards this, we present a novel critical path monitor based dynamic voltage scaling scheme. Considering both process and temperature variations, it minimizes the operating power under a given timing error probability (TEP) bound. An exact method to decide the optimal switching thresholds is also proposed. Experimental results on 45nm industrial designs show that with only 1% TEP, our scheme can reduce the operating power by up to 75.3% compared with the constant voltage scheme. To the best of the authors' knowledge, this is the very first work on dynamic configuration for graceful degradation in sub-threshold designs.