This study investigated the fermentation and quality characteristics of a fermented beverage, prepared by semi-anaerobic culture, using sea tangle extract. A central composit design using alcohol(0, 0.5, 1.0, 1.5, 2.0% [all v/v] ), sugar(0, 5, 10, 15, 20% [all w/v] ) and $65^{circ}Brix$ citrus juice(0, 1.0, 2.0, 3.0, 4.0 % [all v/v] ) was used to find the optimal mix for fermentation. Sensory characteristics, such as color, flavor, taste, sweetness, saltiness, sourness and overall quality, were measured using a response surface methodology computer program. The optimal conditions that produced the highest acidity of 0.94 were 2.0 % ethanol, 10.17 % sucrose and 1.99 % citrus juice. The optimal conditions that produced gel 20.13 nun in thickness were 1.98% alcohol, 10.94% sucrose and 1.62% citrus juice. The overall optimal conditions that satisfied all the sensory requirements for a sea tangle beverage were 1.0% alcohol, 10.0% sucrose and 4.05% citrus juice.