Sub-Terahertz 303 GHz rectennas of Microstrip line (MSL) and Coplanar waveguide (CPW) was developed and measured using a 300 kW-class Gyrotron. Measurement of Microstrip patch antenna was carried out using finline which is MSL-to-waveguide transducer and gain of 8.32 dBi was obtained. CPW rectenna was developed by the photolithography process. As a result, the maximum output power and rectenna power density (output power per unit area) for MSL rectenna and CPW rectenna were 17.1 mW, 3.43 kW/m2, and 14.5 mW, 4.87 kW/m2, respectively. The RF-DC conversion efficiency was obtained 2.17 % with 342 mW input power and 130 Ω DC load [1].