The classification of power systems operating states plays an important role in power systems control and operation. Determining the state of a power system is crucial and requirements for real-time decision making in power systems security assessment demand low dimensionality and low computational time. This paper investigates the performances of feature extraction based on mutual information in power system state classification with machine learning. The AdaBoost algorithm is used for classification based on large training databases and feature extraction is applied in order to reduce their dimensionality.