This paper is concerned with backward problem for nonlinear space fractional diffusion with additive noise on the right-hand side and the final value. To regularize the instable solution, we develop some new regularized method for solving the problem. In the case of constant coefficients, we use the truncation methods. In the case of perturbed time dependent coefficients, we apply a new quasi-reversibility method. We also show the convergence rate between the regularized solution and the sought solution under some a priori assumption on the sought solution.Keywords: Inverse problem for fractional heat equation, truncation method, approximate solu-2 recently considered by Tuan and Nane [19].Next we give some details about our methods for the following two cases The first case: a(t) = 1. First, we transform the problem (1) into a nonlinear integral equation, then we apply the Fourier truncation method(using the eigenfunctions cos(px), p = 0, 1, 2 · · · of the Laplacian in the interval (0, π) with Neumann boundary conditions) associated with some techniques in nonparametric regression to establish a first regularized solution U Mn,n (x, t) which satisfies (33). To obtain the estimate between U Mn,n and u, we need some stronger assumptions on u, such as (27) and (29). The main result is Theorem 2.6. However, as pointed out in Remark 2.5, the assumptions (27) and (29) are difficult to come up in practice. Motivated by this, when g = 0 we develop a second regularized solution U Mn,n defined by (69) to obtain the estimate for u ∈ C([0, T ]; H γ (Ω)) (see Remark 2.5 for more details). The main result for the second type of regularization is given in Theorem 2.8. It is important to realize that the second regularized solution is a modification of the first regularized solution. Our methods in this paper can be applied to solve many ill-posed problems of nonlinear PDEs such as Cauchy problem for nonlinear elliptic, nonlinear ultraparabolic, nonlinear strongly damped wave equations, and many others. The second case: a(t) depend on t and is perturbed. Note that if the coefficient a in the main equation of (1) is not noisy then the Fourier truncation method in [21] can be applied to Problem (1). However, the difficulty occurs for (86) when the time dependent coefficient a(t) is noisy. Indeed, we assume that a(t) is noisy by observed random data a(t) which satisfy thatwhere ξ(t) is Brownian motion. If we have used a Fourier truncation solution for (1), then the regularized solution would contain some terms such as exp p 2β T t s t a(τ )dτ ds , which would lead to some complex computations. Hence, we don't follow the truncation method as in [21], instead we develop a new method to find a regularized solution. We will apply a new quasireversibility method for solving the problem. Further details of this method can be found in Tuan [21]. In this case our main results are Theorems 3.2 and 3.7.In this paper, we only study the upper bound of the convergence rate. In a future work, we will study the minimax rate of convergence for fi...