A novel method for optimal placement of distributed generations based on sensitivity analysis has been proposed in this paper. The proposed method takes the minimization of active power losses of distribution network as optimization objective, and determines the optimal location of distributed generations by order of the sensitivities of power losses respective to nodal loads, then calculates the optimal capacities of distributed generations by the solution of a set of linear algebraic equations which is derived by use of the mathematical extremum principle. The proposed method is clear and brief, very fast in computation. This method has been tested on IEEE 33-bus distribution system, and the simulation results has shown that the proposed method is efficient.