Extensive studies on WAMS based fault location technique are based on the idealized conditions of the measurement, so that they are limited in the actual applications in electric power system. In order to make the power grid to be provided reliable fault location technique on incomplete information conditions such as extreme weather, communication failures, etc., an effective fault location technique for transmission grids based on fault observability model is presented. High accuracy in fault location is achieved by using an accurate distributed parameter line model for the transmission lines. The performed work is provided with high performance which is featuring robustness to the uncertainties in fault inception angle, load current and fault path resistance. The transient fault data is obtained from PMUs in the proposed technique, and it does not require any information of the source impedance. This novel fault location technique does not need to be solved in an iterative manner and the calculation is fast. The validity and robustness of the proposed algorithm is evaluated by many simulations. This work is supported by National Natural Science Foundation of China (No. 51377068).