An electro-optic modulator includes a semiconductor substrate, a core region and slab regions. The core region is formed on the semiconductor substrate and includes a first low concentration doping region, a second low concentration doping region and a high concentration doping region formed between the first low concentration doping region and the second low concentration doping region. Slab regions are formed on the semiconductor substrate. The slab regions are in contact with the core region. A width of a depletion region included in the core region may be controlled based on a reverse bias voltage applied between the first low concentration doping region and the second low concentration doping region. If the width of a depletion region is controlled, an optical signal transmitted through the depletion region may be controlled. The electro-optic modulator according to example embodiments may increase an operation speed and decrease a power consumption of a system.