Measurement noises and the singularity in matrix numerical computation bring errors in short term load forecasting. To reduce the influence of noises on initial values， Ensemble prediction in numerical weather prediction is thereafter used in short term load forecasting. In chaotic phase space reconstruction forecasting， level noises with normal distribution are added to the reference vector to form many reference vectors with perturbations. Many forecasting results are respectively given based on the different reference vectors with perturbations to obtain the probabilistic forecasting result. The higher accuracy and probabilistic characteristic are two main advantages rendered by the Ensemble technique of chaotic forecasting.