In this paper, a smooth Gaussian radiator from a rectangular waveguide TE 10 mode to a circular waveguide HE 11 mode for W-band is proposed. The W-band smooth Gaussian radiator consists of two parts: one is to convert the TE 10 mode in the rectangular waveguide into the TE 11 mode in the circular waveguide using a square-circle transition; the other part is the TE 11 mode converted into a Gaussian distributed HE 11 mode using the waveguide wall smooth radius structure. The simulation results show that conversion efficiency of TE 10 -TE 11 is over 99.6% from the 84GHz-104GHz. The coupling efficiency of the TE 11 -HE 11 is more than 96% in the frequency range of $92\text{GHz}\sim 98\text{GHz}$. Meanwhile, the energy transmission efficiency can reach more than 98%. The smooth Gaussian radiator has a simple structure, easy to machine, and avoids the disadvantage of the corrugated Gaussian radiator.