Fractional order systems(FOS) have attracted much attention since it plays an important role in many fields of science and engineering. Fractional order systems have many relationship with integer order systems(IOS). Can a fractional order system be replaced with an integer order system in the sense of solution equivalence? This paper deals with the relationship between FOS and IOS. It is concluded that the research of fractional order systems is necessary and its properties can not be replaced by any other system. The stability and composite derivative of FOS are discussed. Some mistake criteria of fractional order systems are pointed out by counterexamples.