We consider a closed oriented surface of genus at least 2. For any positive integer k, an essential closed curve on the surface with k self-intersections is called a k-curve. A pair of curves on the surface are said to be k-equivalent, if they have the same intersection numbers with each k-curve. In this talk, I will discuss the general picture of a pair of k-equivalent curves and the relation between k-equivalence relations for different k’s. This is a joint-work with Hugo Parlier