Response surface methodology was used to investigate the effect of ultrasound-assisted extraction conditions including solvent:seaweed ratio (10:1-30:1), extraction temperature (30-50 degrees C), and extraction time (30-60 min) on the total phenolic compounds and antioxidant activity of red seaweed (Laurencia obtuse) extracts. The optimum extraction parameters for maximum phenolic content were as follows: solvent:seaweed ratio, 30:1; extraction temperature, 50 degrees C; and extraction time, 42.8 min. The experiment results showed that the solvent:seaweed ratio was the most significant parameter for the extraction. The optimum extraction conditions for maximum antioxidant activity were as follows: solvent:seaweed ratio, 24.3:1; extraction temperature, 45.3 degrees C; and extraction time, 58 min. Under the above-mentioned conditions, the experimental total phenolic content and antioxidant activity value were 26.23 +/- 0.75 mg GAE/g seaweed and 120.89 +/- 0.81 TEAC, respectively, of seaweed tested, which are well compatible with the predicted contents.