A sequential machine is said to be k -controllable if there exists a minimum integer k such that every state transition can be achieved by an input sequence of length k . In this paper properties of controllable sequential machines are discussed. The main results are as follows: (1) An efficient procedure for controllability test is given. (2) It is shown that every uncontrollable strongly connected machine has an autonomous component machine. (3) The upper bound of k in a controllable machine containing a loop of length l is given. (4) The upper and lower bounds of k in k -controllable machines are shown. (5) It is shown that any sequential machine can be realized by a controllable or uncontrollable machine.