In this paper, the rate of heading consensus of a group of agents governed by Vicsek's model is estimated when the initial headings of the agents are all confined in (−π/2, π/2). Then, under a well-known connectivity assumption, a necessary and sufficient condition for Vicsek's model to guarantee the global heading consensus of the group is presented. Finally, we extend the result on Vicsek's model to a class of consensus protocols, which includes the popular averaging protocol as a special case.