After independence, South Sudan back into conflict is prolonged. The conflict in this new state trigger by internal political rivalry between President Salva Kiir Maryadit and former Vice President Riek Machar Teny Dhurgon which then extends to the ethnic divisions. The civil war that occurred in 2013 have led to thousands of civilians were killed and hundreds displaced. The growing conflict for approximately one and a half between the Government of South Sudan led by President Salva Kiir and rebels Maryadit by Riek Machar Teny Dhurgon can not be resolved through mediation efforts by IGAD. IGAD is a subregional African organizations that play a role as mediator in the settlement of the conflict in South Sudan. In this case, IGAD seeks to bring together and make both parties agree to end the conflict. But the mediation efforts undertaken IGAD bring the two sides is difficult to reach a peace deal. Therefore, this research focuses on the causes of the failure of IGAD as a mediator in mediating the conflict in the country, with a research question Why IGAD as a mediator failed in reaching a peace agreement in South Sudan conflict? using the concept of International Organizations and Conflict Resolution theory, this research seeks to analyze the causes of failure of IGAD as a mediator in South Sudan conflict that occurred in 2013. The results showed that the inability to run the IGAD in the extraction and termination functions as a mediator in the mediation process in because of the involvement of various actors both regionally and internationally as well as the presence of partiality in the negotiations led to the collapse of IGAD in ending the conflict towards a peaceful settlement.