Unsupervised learning is a discipline of machine learning which aims at discovering patterns in big data sets or classifying the data into several categories without being trained explicitly. We show that unsupervised learning techniques can be readily used to identify phases and phases transitions of many body systems. Starting with raw spin configurations of a prototypical Ising model, we use principal component analysis to extract relevant low dimensional representations the original data and use clustering analysis to identify distinct phases in the feature space. This approach successfully finds out physical concepts such as order parameter and structure factor to be indicators of the phase transition. We discuss future prospects of discovering more complex phases and phase transitions using unsupervised learning techniques.Classifying phases of matter and identifying phase transitions between them is one of the central topics of condensed matter physics research. Despite an astronomical number of constituting particles, it often suffices to represent states of a many-body system with only a few variables. For example, a conventional approach in condensed matter physics is to identify order parameters via symmetry consideration or analyzing low energy collective degree of freedoms and use them to label phases of matter [1].However, it is harder to identify phases and phase transitions in this way in an increasing number of new states of matter, where the order parameter may only be defined in an elusive nonlocal way [2]. These new developments call for new ways of identifying appropriate indicators of phase transitions.To meet this challenge, we use machine learning techniques to extract information of phases and phase transitions directly from many-body configurations. In fact, application of machine learning techniques to condensed matter physics is a burgeoning field [3][4][5][6][7][8][9][10][11][12][13][33]. For example, regression approaches are used to predict crystal structures [3], to approximate density functionals [6], and to solve quantum impurity problems [10]; artificial neural networks are trained to classify phases of classical statistical models [13]. However, most of those applications use supervised learning techniques (regression and classification), where a learner needs to be trained with the previously solved data set (input/output pairs) before it can be used to make predictions.On the other hand, in the unsupervised learning, there is no such explicit training phase. The learner should by itself find out interesting patterns in the input data. Typical unsupervised learning tasks include cluster analysis and feature extraction. Cluster analysis divides the input data into several groups based on certain measures of similarities. Feature extraction finds a low-dimensional representation of the dataset while still preserving essential characteristics of the original data. Unsupervised learning methods have broad applications in data compression, visualization, online advertising and reco...