Once the exclusive domain of the world’s navies and coast guards, maritime security field now involves a complex web of private and public players. Somali piracy has been the key factor contributing to this development. By adapting their tactics and stretching their geographical reach, Somali pirates have demonstrated the limits of state security provision at sea, leaving the shipping industry and government regulators few alternatives but to adopt self-protective measures and accept the use of Privately Contracted Armed Security Personnels(hereinafter PCASP). PCASPs have grown to serve as protectors of roughly two-fives of the ships travelling in the High Risk Area exposed to Somali piracy, and appear to have reduced the success rate of pirate attacks. The immense reduction in piracy off the coast of Somalia indicates that PCASP are having a positive impact in eliminating the piracy problems and reducing the risks of transit in the area. In my opinion, their importance appears set to increase in the near future. The deployment of PCASP on board vessels, though gaining wider acceptability, is still being approached with great caution, largely due to the legal and regulatory complexities involved and outpacing regulation. Currently, the legal framework relating to use of Private Maritime Security Company(hereinafter PMSC) and PCASP on board protecting ships from Somali-based piracy is complex, sometimes ambiguous or inconsistent, and currently in a state of flux. Moreover, some incidents and critics of PCASP raise the issue of jurisdiction, the applicable national laws in an encounter, the international guidance on the use of PCASP, etc. Therefore, there is a myriad of possibilities regarding legal problems, but they tend to be overlooked because of the unexpectedly high success rate of their protective activities against pirates. As it stands, there has been no valid endorsement for institutionalization by the International Maritime Organization and other major international organizations and no legally bing instruments agreed under international law. This article has set forth the legal issues, obligations, and the potential responsibilities relating to the deployment of PCASP on board ships to counter piracy. In particular, this article focuses on the legal issues including jurisdictional questions of the incident involving PCASP, legal issues relating innocent passage of PCASP aboard merchant vessel, restrictions governing use of force by PCASP in self-defense, and international responsibility of flag State involving the wrongful or indiscriminate use of force by PCASP.