Overtaking is one of the common actions that the driver performs while travelling on the road. Without enough information about the road, overtaking is dangerous and causes many accidents. Although overtaking is important, little research was done in this area. Especially, there is no research focus on helping the driver to judge whether it's safe enough or not to do overtaking. The purpose of this paper is to provide overtaking assistant service to help the driver to do the overtaking safely. From the proposed overtaking decision assistant figure, the driver can easily determine whether the overtaking is safe or not. However, since the GPS receiver's error will influence the trajectory calculation, it has to be considered to improve the original overtaking assistant service.