In this paper we propose a couple of fractional order observers for nonlinear commensurate fractional order systems i.e. a reduced order observer and a fractional Luenberger observer in the algebraic and differential setting. We introduce the Fractional Algebraic Observability (FAO) property, like a measurement degree of fractional observability of states variables. Finally a comparison between two observers illustrates the effectiveness of the suggested approach, this is performed with two different numerical examples: a linear mechanical oscillator with integer and fractional order damping; and a nonlinear fractional order Duffing System.