This paper presents a microwave reflectometer using self-mixing oscillator antennas. A unique characteristic of the proposed reflectometer is its simple structure without a circulator and a directional coupler. Each self-mixing oscillator antenna utilizes a Gunn diode as an active device and a rectangular waveguide as a resonator and a radiator. The determination of dielectric properties of materials was carried out at 9.79 GHz and the measured results using the proposed reflectometer were in good agreement with those by a dielectric probe. In addition, the reflectometer has several industrial applications, e.g. fruit classification, moisture content sensing, etc.