This paper presents a method of determining the best location of load shedding considering transient voltage stability. In the differential-algebraic model of a power system， the trajectory encounters the situation of the impasse surface corresponding to the system leading to voltage instability. Voltage collapse would be prevented by moving the impasse surface far away from the trajectory through load shedding. The distance the impasse surface moves is defined as an index of the effectiveness of control measures. The sensitivities of the index with respect to bus load are calculated， according to which the best location of under-voltage load shedding can be selected. Simulations on the IEEE 39-bus system have proved that the determination of the location of load shedding by the sensitivity method proposed is rational and effective.