This research discusses about Singapore's Reasons for Not Approved Timor Leste's Membership in ASEAN in 2011-2017. The purpose of this research was to find out the factors that underlying Singapore in making that decision. Based on the research that has been done, the authors find that the thing that underlies Singapore has not agreed to East Timor membership in ASEAN until 2017 is because Singapore as a country that plays an active role in ASEAN has concerns if Timor Leste joins ASEAN, Timor Leste whose their economic condition is lower than the other ASEAN members, it will affect ASEAN stability, which if the stability of ASEAN is disrupted a little more will also have an impact on Singapore's domestic conditions.