In this note, we consider a new unknown inputs observer design for nonlinear systems. The main idea consists in determining the estimation error and theorem mean value parameters to introduce them into the proposed observer structure. This process is designed on the basis of mean value theorem and genetic algorithm. This observer does not use LMI technique for stability study.The stability study relies on the use of quadratic Lyapunov function. Two numerical examples are provided to show the performance of the proposed approach.KEYWORDS genetic algorithm, mean value theorem, nonlinear system, state estimation, unknown input observer 1904