The　atom-bond　connectivity　(ABC)　index　of　a　graph　G,　is　defined　as　the　sum　of　the　weights　(d　u+d　v-2/　d　ud　v)　1/2　of　all　edges　uvof　G,　where　d　u　denotes　the　degree　of　a　vertex　uin　G.　The　ABC　index　provides　a　good　model　for　the　stability　of　linear　and　branched　alkanes　as　well　as　the　strain　energy　of　cycloalkanes.　The　sharp　lower　bounds　on　ABCindex　of　chemical　unicyclic　graphs　is　obtained.