The present study reports on the extraction of antioxidant compounds from Chlorella vulgaris. Multiresponse surface methodology coupled with a central composite design was used to optimize the independent parameters for ultrasonic-assisted extraction (UAE). Four independent parameters were optimized, including extraction time (15-155 min), ethanol volume (35-95%, v/v), extraction temperature (15-75 degrees C) and liquid/solid ratio (30-150 ml g(-1\)). Total phenolic content and antioxidant capacity [2,2'-azinobis(3-ethylbenzothiazoline-6-sulphonic acid) and chromiumreducing antiaoxidant capacity methods] were considered response parameters, whereas the effects of extraction time, ethanol volume, extraction temperature and liquid/solid ratio were studied. The optimal condition was obtained with an extraction time of 146 min, ethanol volume of 71% (v/v), extraction temperature of 72 degrees C and liquid/solid ratio of 62 ml g(-1) of sample. The experimental values agreed with predicted values within a 95% confidence level. This indicates that multiresponse surface methodology is suitable for optimizing UAE of antioxidant compounds from C. vulgaris.