A transition structure from microstrip to waveguide, based on Land Grid Array(LGA) package technology, is designed in this paper. The structure consists of microstrip to stripline transition and stripline to waveguide transition. The metal waveguide shorter is no needed in the transition, thus the manufacturing process is simplified and the standard waveguide can connect with this transition directly. Simulation result shows that the minimal insertion loss is 1.85 dB and the return loss is less than -10 dB in 122-140 GHz. In addition, this transition can be integrated with other chips and passive devices. Therefore, the transition is useful for the improvement of property of RF microsystem.