The United Nations has failed to carry out security sector reform in Timor Leste. This failure has resulted in a violent conflict between the police and the army in 2006-2008, unprofessional state security institutions, and low public trust in state security institutions. However, at the same time, the people feel the security of their environment which is getting better and high trust in informal institutions as a security reference. Previous studies have been able to explain the failure of the UN in Timor Leste by criticizing the liberal approach in security studies. Similar criticism was carried out in this study but by using an institutionalism approach regarding the relationship of informal and formal institutions in the formation of state security institutions. This study aims to determine how the role of informal security institutions (Falintil veterans, uma lisan, and churches) in the security sector in Timor Leste and its relationship with state security institutions. This research has used explanatory sequential methods with primary data derived from interviews, questionnaires, observations in Dili and Ermera as well as secondary ones from previous studies. This research has found the role of Falintil veterans as a toxic institution whose function is to weaken state security institutions by nepotism and clientelism. While the role of the church and uma lisan are as tonic institutions that strengthen the state security institutions by providing security in the realm of cognition and problem-solving mechanisms outside the state court. This finding recommends to the United Nations and the government of Timor Leste to strengthen the role of informal tonic institutions and place them in coexistence relations with the state security institutions.