In this paper we consider a final value problem for a diffusion equation with timespace fractional differentiation on a bounded domain D of R k , k ≥ 1, which includes the fractional power L β , 0 < β ≤ 1, of a symmetric uniformly elliptic operator L defined on L 2 (D). A representation of solutions is given by using the Laplace transform and the spectrum of L β . We establish some existence and regularity results for our problem in both the linear and nonlinear case.where ϕ is a given function. Here J is the interval (0, T ), The notation c D α t for 0 < α < 1 represents the left Caputo fractional derivative of order α which is defined by c D α t v(t) := I 1−α t (N.H. Tuan) Applied