This thesis is concerned basically with establishing theoretical background for the Riemann-Liouville definition of fractional differentiation and integration of the function of several variables and solving certain type of fractional partial differential equations using some approximated methods. Banach fixed point theorem has been used to prove the existence and uniqueness of a solution to this type of equations. Two weighted residual methods (collocation and Galerkin) have been used to treat these equations approximately, where a new formulation for the two dimensional polynomial approximation has been established and used to approximate the unknown function. Also, a proposition has been proved to find a general result for the fractional derivative of this approximation. This general result has been used to construct new simple, but efficient, method called polynomial approximation method. Moreover, the convergence and stability of all the three approximated methods have been investigated. Finally, a program for each one of these methods, has been written with the aid of MatLab (v. 6.5) in order to take the whole benefit of these techniques.