The consensus problem of heterogeneous multi-agent systems composed of first-order agents and second-order agents in directed graph is investgated. A linear consensus protocol is proposed for solving such a consensus problem. The sufficient conditions for achieving consensus are established by using the graph theory and matrix theory in fixed and switching topology respectively, and these conditions are dependent on control gain and communication topology. Consensus equilibrium point is derived in the fixed topology. It is proved that only root nodes in the interaction topology can make contributions to the equilibrium point. Numerical examples are provided to prove the correctness of the theorems.