Stanley introduced the notion of chromatic symmetric function for any graph and Stanley-Stembridge conjectured that chromatic symmetric function expands positively in terms of elementary symmetric functions for any (3+1)-free graph. Shareshian-Wachs refined this conjecture by introducing a q-analogue called chromatic quasisymmetric function. In this talk, I will give an explicit inductive formula for elementary symmetric function expansion of chromatic quasisymmetric function for any unit interval graph, which in particular proves the Stanley-Stembridge conjecture.