Recently, microalgae have become important in their health, and cosmetic applications since they are viewed as new sources of carotenoids. Fucoxanthin is also a type of carotenoid. The anti-diabetic, anti-obesity, anti-cancer, and antioxidant properties of fucoxanthin have been widely reported. Since these valuable properties, they also represent a valuable resource of nutraceuticals for functional food applications. This study aims to determine the amount of fucoxanthin, gallic acid, and rutin in Nitzschia thermalis obtained from the Ege University Microalgae Culture Collection. The extraction parameters have been optimized using response surface methodology. The extraction temperature (25, 35, and 45 degrees C), the extraction time (10, 20, and 30 min) and the biomass/solvent ratio (0.005, 0.001, and 0.015 g ml(-1)) have been assessed as response variables in the Box - Behnken design. The amount of fucoxanthin was determined by the C-30 column at 450 nm, while both the amount of gallic acid and rutin were separated in the C-18 column at 275 nm by HPLC-DAD. In the present study, the optimum extraction conditions providing the maximum amount of fucoxantin, gallic acid, and rutin were selected by applying the "desirability" function approach in response surface methodology. Finally, the temperature has been determined to be 27.30 degrees C, the extraction time 10 minutes, and the biomass ratio 0.05 g ml(-1). Under these conditions, the optimum fucoxanthin level has been determined as 5.8702 mg g(-1), the gallic acid level as 0.0140 mg g(-1), and the rutin level as 0.0496 mg g(-1). The findings are in good agreement with international published values for fucoxanthin content. In addition, response surface methodology was shown to be an effective technique for optimising extraction conditions for maximum fucoxanthin yield. In conclusion, these findings may be applied in the development of extraction methodologies for value added microalgea products as well as can serve as a reference for the extraction of fucoxanthin having high gallic acid and rutin from other brown microalgae, and therefore it could potentially be applied in both pharmaceutical and food industries.