The effect of ultrasonic temperature on oil yield was shown in Figure 3. The oil yield increased from 23.60% to 53.54% with increasing the temperature from 20 to 50°C, and then leveled out after 50°C. Temperature affects the performance of ultrasonic pretreatment by influencing the amount of vapor in ultrasonically generated bubbles. The vapor pressure at high temperatures will help to form vapor-filled bubbles. The explosion of those vapor-filled bubbles had an effect named as cushioning effect (Lou et al., 2010). As a result, the cavitational effect would be weakened, and thus the oil yield remained unchanged with continuously increasing the temperature. Therefore, ultrasonic temperature of 50°C was seen to be suitable for ultrasonic pretreatment.