A system and method for identifying a mobile user cluster by call detail records (CDRs) is provided. The system and method identifies at least one mobile user cluster according to a plurality of CDRs generated by a plurality of mobile users during a first period and a second period of time. Each mobile user of the identified cluster generates at least one CDR at a same geographical location during the first period of time, and a mobile user sequence is formed between any two mobile users of the identified cluster. At least one CDR is generated between any two neighboring mobile users of the mobile user sequence during the second period of time. Examples for the mobile user cluster include cohabiting family members, familiar neighborhood, colleagues, schoolmates, etc.