People play different roles in various social networks. Even in a single network, people may interact with others based on different roles, and there are various relationships among them. However, current research usually treats all relationships homogeneously (i.e. friendship). In this paper, we try to identify different types of relationship (family, colleague, and social) within social networks. By analyzing a large-scale cellphone network, we gain insights about human mobility patterns. We design three metrics to capture colocation behaviors for cellphone users, taking spatial-temporal factors into consideration. These metrics show that users with different relationships demonstrate significantly different co-locating patterns. With these metrics as features, we adopt supervised approach to classify cellphone user pairs into different relationship categories. Comparing to using network and communication features, co-location metrics demonstrate better performance to fulfill the task of relationship identification.