Abstract: In order to design precise and feasible ranging method with Impulse Radio Ultra Wide Band (IR-UWB) signal during energy detection, two new Time Of Arrival (TOA) estimation algorithms based on optimal and suboptimal thresholds are respectively proposed. For optimal method, with the relationship between energys statistics in receiver and small-scale attenuation, a closed form of threshold is derived, and the TOA estimation is obtained under the Minimum Mean Square Error (MMSE). For suboptimal method based on optimal threshold analysis, a recursive form of threshold selection using Newton iteration is developed with false alarm probability constraint.Through simulations, compared with other similar algorithms, the optimal method enhances greatly the ranging accuracy, the suboptimal method is easier to implement while its performance reduces less.