We provide an analytical proof of the local stability contained in the region of coexistence of the three species of a tritrophic fractional--order food chain model. An expansion formula for fractional derivatives given as in form of a series involving function and moments of its k-thderivative is used. The tritrophic fractional--order food chain system is converted to a system of ordinary differential equation of order 3M. Also stability analysis is studied by using the fractional Routh--Hurwitz stability conditions in origin. Numerical results show that the presented method is easy to implement and accurateto differential equations of fractional order.