Device-to-device (D2D) communication has attracted a lot of attention since it is considered as a key technology for 5G. The high rate can be obtained by the short distance between D2D users. However, this superiority will be lost when the distance between D2D users increases, since the transmit power of user is limited. Motivated by these, in order to improve the performance of D2D link, this paper investigates the D2D communication assisted with another D2D user working as a relay node. We consider three modes including traditional D2D communication, amplify-and-forward (AF) relay node assisted D2D communication and D2D communication aided by decode-and-forward (DF) relay node. In this paper, we analyze the achievable rate of each mode, and propose a mode selection scheme which is presented with closed-form expressions， which include only elementary functions. The mode selection scheme guarantees the D2D link always achieves the maximum rate of three modes. Numerical results show the significantly enhanced performance with the mode selection scheme.