A family of powerful and resource efficient Turbo codes is developed for sensor networks. In the hardware realization of Turbo code decoder, recalculation of the forward or backward state metrics is an effective method to reduce the size of state metrics cache. To facilitate recalculation of the state metrics, an optimized max∗ operator is presented in this paper. Calculation of the proposed max∗ operator includes two correction terms, and each one is approximated by using simple comparing, shifting and addition operations. Moreover, approximation of the proposed max∗ operator is specially optimized for reverse calculation. Although there is small error in the reverse calculation, it has been shown that the maximum error is about three percent, and the proposed approximation is validity for reverse calculation.