In this paper, we study variational structure for the (k, ψ) -Hilfer fractional derivative operator. We investigate some properties of (k, ψ) -Hilfer fractional integral and derivative which are used of variational structure. Also, using variational methods and critical point theory, we study the existence of solutions to a problem with (k, ψ) -Hilfer fractional derivative. Our work extent and improve some recent works in the literature.