In this paper, we consider the nonlinear fractional Schrödinger equations with Hartree type nonlinearity. We obtain the existence of standing waves by studying the related constrained minimization problems via applying the concentration-compactness principle. By symmetric decreasing rearrangements, we also show that the standing waves, up to a translations and phases, are positive symmetric nonincreasing functions. Moreover, we prove that the set of minimizers is a stable set for the initial value problem of the equations, that is, a solution whose initial data is near the set will remain near it for all time.( 1.3)The fractional Schrödinger equation plays a significant role in the theory of fractional quantum mechanics. It was formulated by N. Laskin [14][15][16] as a result of extending the Feynman path integral from the Brownian-like to Lévy-like quantum mechanical paths. The Lévy processes, occuring widely in physics, chemistry and biology, lead to equations with the fractional Laplacians which have been recently studied by [1][8] [23]. When α = 1 2 , NLS (1.1) can be used to describe the dynamics of pseudo-relativistic boson stars in the mean-field limit, see [7]. When α = 1, the Lévy motion becomes Brownian motion and the fractional Schrödinger equation turns to be the well-known classical nonlinear Schrödinger equation which has been studied by many authors, see for instance [2][3][17] [21].2000 Mathematics Subject Classification. 35Q55.