  Fractional variational approach has gained much attention in recent years.
There are famous fractional derivatives such as Caputo derivative, Riesz
derivative and Riemann-Liouville derivative. Several versions of fractional
variational principles are proposed. However, it becomes difficult to apply the
existing fractional variational theories to fractional differential models, due
to the definitions of fractional variational derivatives which not only contain
the left fractional derivatives but also appear right ones. In this paper, a
new definition of fractional variational derivative is introduced by using a
modified Riemann-Liouville derivative and the fractional Euler-Lagrange
principle is established for fractional partial differential equations.
