Abstract: Social network theory is used to mathematically model massive transactions for aiding law enforcement agencies, in the status of art of increasing rate of gang crimes which are more dangerous than common crimes. Graph-based clustering algorithms are employed to mine out gangs of transactions, to analyze their structures and smartly identify gangs engaged in illegal transactions. Thereby, it provides for accurate crime clues for law enforcement agencies, and enhances initiative and efifciency to crack down on criminal offenses.