Recently, there has been considerable research on ITS(Intelligent Transport System). The platooning of some trucks on the express way is discussed widely as an important part of ITS application. In order to organize the platooning, it is required to converge each other. However, because the sequence of converging from wide area depends on the driver's decision and there is no basis velocity for converging, it is potentially difficult to achieve efficient convergence for platooning. In this paper, we propose a velocity decision algorithm for platoon and it has two main components. The one is decision the sequence of converging and the other is call into account whether a platoon relevant truck can converge each others. We implemented and evaluated our proposition in comparison to simple converging method by computer simulation. As the result, we show that our proposal have efficacy as converging for platooning.