In fusion reaction, plasma parameters such as the density and temperature of electrons should be diagnosed continuously. There are various methods to diagnose plasma parameters. In these, reflectometry systems are widely used to measure the electron density and plasma physics study. In reflectometry systems, antenna plays an important role as a transmitter/receiver element. This paper presents the design of a D-band (110-170 GHz) corrugated horn antenna suitable for reflectometry system. The simulated results for antenna are compared with that of the measurements. Further, different structures are proposed to ease fabrication complexities and reduce cost.