This paper investigates the distributed consensus problem of fractional-order multi-agent systems under a time-invariant communication topology, where the dynamics of each agent is described by a general fractional-order differential equation. To achieve consensus, a fractional-order observer-type consensus protocol based on relative output measurements is introduced. By using tools from Lyapunov stability theory for fractional-order systems, two theorems about the consensus of fractional-order multi-agent system with a fixed communication topology having a spanning tree are then proposed. Finally, the effectiveness of the theoretical results is demonstrated through numerical simulations.