Abstarct: Abstract This thesis proposes a novel observer for a class of certain fractional-order affine nonlinear system. To achieve this goal, the differential mean value theorem method is utilized in the observer design, where the difference between a vector function at two inputs, is replaced by the linear difference of these two values. Consequently, the nonlinear dynamics of the fractional order system are replaced by the linear parameter variable dynamics. This property is one of the major advantages of using the mentioned method in the observer design. Stability of the observer dynamics is proved using the Lyapunov theorem, resulting in some linear matrix inequalities whose solution yields the observer gain matrices or vectors. To demonstrate the effectiveness and satisfactory performance of the proposed observer, two numerical simulation examples are presented. Furthermore, a comparative analysis is conducted to evaluate the performance of the proposed observer compared to another observer applied on a fractional order nonlinear system. The results demonstrate that the proposed observer achieves faster convergence of the estimated states to the actual states.