A uni-planar, easy for integration and compact antenna for millimeter wave applications is presented in this paper. The concept of metamaterials is employed to realize the antenna miniaturization. For the designed antenna, a small rectangular semi-loop performs as the radiating element. Metamaterial-inspired parallel strips are introduced that are coupled to the original antenna. It is found that, with this modification, the total size of the antenna is reduced and the impedance matching of the whole antenna system is improved. To further show the impact of ground plane size, the proposed antenna is simulated with different ground plane sizes. In addition, by using the Wheeler-cap method and fullwave electromagnetic simulations, it is found the proposed antenna exhibits high radiation efficiency at the working frequencies.