Abstract: For a class of fractional order system which has similar structure to the traditional first order inertia link transfer function,the scope of the parameters which can ensure the stability of such fractional order system is given and the relation-ship between time response and the fractional order is analyzed.Then both the fractional order controller PI^λ and integer order controller PI are designed based on such kind of fractional order system.The parameters of the controllers are obtained by particle swarm optimization algorithm.Results show that both the controllers can achieve satisfactory performance,so the methods proposed in this paper are effective.Considering that the integer order controller PI is more simple and easy to realize compared with the fractional order controller PI^λ,therefore the controller PI is preferential when controlling such kind of fractional order object in engineering practice.