This paper investigates the circular formation control problem of multiple nonholonomic vehicles of unicycle type. The measurement of each vehicle is based on its local coordinate frame and the communication network among vehicles is modeled by a directed cycle graph. A distributed dynamic control law is designed by only using the local measurement of each vehicle and information of its neighbors in the network. The proposed control law guarantees that all vehicles move with a prespecified angular velocity along a common circle with the given center and radius, and maintain evenly spaced along the circle. Furthermore, the velocity constraint including positive-minimum linear velocity of each vehicle is explicitly taken into account. (C) 2016 Elsevier Ltd. All rights reserved.