Within the past decade, there has been an increased interest in utilizing social impact bonds (SIBs) to finance social service initiatives. SIBs are result-oriented public-private partnerships that rely on private capitals to deliver public services. While some SIBs have successfully generated investor returns, others have been terminated in earlier stages. By analyzing three case studies in Peterborough UK, New York and Chicago, USA, this paper investigates the factors that contribute to the success of SIBs. The results indicate that comprehensive services, evidence-based program, and early intervention are the key to successful SIBs.