The optimal formation configuration for underwater localization of autonomous underwater vehicle (AUV) with a more general measurements model is investigated. The evaluation function is derived on the basis of Cramer-Rao lower bound (CRLB) or the corresponding Fisher information matrix (FIM). We determine the optimal formation configuration for the regular placement and the linear placement patterns, respectively. Subsequently, the influence of the measurements model on the optimal formation and the sensitivity of the localization performance to the factors, separation angle and range, are also presented. The simulation results demonstrate that the localization performance is more sensitive to the range rather than to the separation angle.