The global demand on essential oil derived from citronella grass (Cymbopogon nardus) leaves as fragrances, aroma and remedies ingredients has been steadily increasing towards commercial scale for decades. Therefore, a study to enhance citronella grass essential oil production capacity through ultrasound pre-treatment is performed. This study investigated the effect of various operating parameters, such as leaf particle size (ground, 5 mm and 10 mm), ultrasound pre-treatment time (10, 20 and 30 min) and the leaf particle mass to extracting water volume (SW) ratio (1:4.0 to 1:12.0) on the essential oil yield. The chemical and physical properties of the citronella oil, namely bioactive constituents, specific gravity, refractive index and colour, were also determined. The experimental results indicate that increased ultrasound pre-treatment time and S/W ratio promoted to the enhancement of oil yields by up to 56.94 % and 19.59 %. The GC-MS analysis proved that citronella oil contains mainly citronellal, geraniol, neral and geranial. Physical properties evaluation also revealed that the specific gravity, refractive index and colour of the extracted citronella oil were of acceptable ranges. The results indicated that the increase of S/W ratio and ultrasound pre-treatment time potentially increase the extraction yield. This study suggests that the combined ultrasound pre-treatment and hydro-distillation is a promising method for extracting high quality citronella oil and could be implemented to scale-up the citronella oil production capacity.