The present study is conducted to investigate the effect(s) of ultrasonic pretreatment on the extraction of essential oils from dried leaves of lemongrass (Cymbopogon flexuosus) by hydro-distillation. The drying process was carried out within a forced convective dryer at 50 degrees C and a drying air velocity of 1 m/s. Using a response surface methodology (RSM) based on central composite design with the face centered, the obtained optimum conditions were as follows: an ultrasound pretreatment for 25 min within a power of 250 W, at 50 degrees C. Modeling showed that all factors (time, power and temperature) were found to be highly significant in the extraction process (p < 0.001), to significant in all 2-way interactions (p < 0.05) except in the Time x Temperature interaction. Then, a second-degree model with interactions was retained. The maximum amount of essential oils was obtained under optimized conditions approximate to 3.093 g/100 g d.m.). The major component of the extracted essential oils is citral, which represents 85% of the extracted mass and it is present in both cis-isomer (neral) and trans-isomer (geranial) (respectively at 38.89% and 45.86%).