We are concerned with the fractional order equations with Hartree type nonlinearity and its equivalent integral equations. We rst prove a regularity result which indicates that weak solutions are smooth. Then, by applying the method of moving planes in integral form, we prove that positive solutions of integral equations are radially symmetric about some point and derive the explicit form of solutions. As a consequence, we also derive the best constants in the corresponding Hardy-Littlewood-Sobolev inequalities.