This work is concerned with unsteady natural convection heat and mass transfer of a fractional MHD viscoelastic fluid in a porous medium with Soret and Dufour effects. Formulated boundary layer governing equations have coupled mixed time space fractional derivatives, which are solved by finite difference method combined with L1-algorithm. Results indicate that the Dufour number (Du), Eckert number (Ec), Soret number (Sr) and Schmidt number (Sc) have significantly effects on velocity, temperature and concentration fields. With the increase of Du (Sr), the boundary layer thickness of momentum and thermal (concentration) increase remarkably. The average Nusselt number declines with the increase of Du and Ec. The average Sherwood number declines with the increase of Sr, but increases for larger values of Sc. Moreover, the magnetic field slows down the natural convection and reduces the rate of heat and mass transfer. The fractional derivative parameter decelerates the convection flow and enhances the elastic effect of the viscoelastic fluid. (C) 2016 Elsevier Ltd. All rights reserved.