Abstract: A novel waveguide-microstrip back to back transition structure by utilizing Quasi-yagi Antenna is introducted in this paper. By adopting high dielectric substrate and adding numbers of the Yagi antenna director, electromagnetic coupling effect between antenna and waveguide is improved to make a smoother transition in passband. Compared with traditional waveguide-microstrip structure, it has a advantagment of wider bandwidth (nearly 68%). Through electromagnetic software CST, a full-wave analysis in the whole X-band bring about better S21(S210.7 dB). The a toralance Analysis to this transition structure Shows that S21 still keeps fine consistency and comparatively low insert loss within deviation of 10%, which can meet the requirement of production.