In this article, the effects of liquid properties and operating conditions on gas-liquid mass transfer under ultrasound irradiation and mechanical stirring were studied and compared. Response surface methodology (RSM) was utilized for the design of experiments and evaluation of the influence of operating parameters. The maximum value of volumetric mass transfer coefficient (kLa) was found to be 0.0714 s(-1) when the ultrasonic horn was located horizontally just above the gas sparger in the tank. Ultrasonic power and the position of ultrasonic horn were found to be the most significant parameters that influence kLa. Also, three empirical correlations were developed to estimate kLa considering liquid viscosity