Essential oil from rosemary (Rosmarinus officinalis L.) leaves was isolated by conventional hydrodistillation using hydromodulus of 1/15 m/v and by distillation after the plant material pretreatment with 10% v/v aqueous solution of natural deep eutectic solvent glyceline. Qualitative and quantitative composition of the oil was determined by GC/MS and GC/FID analyses, while its antioxidant activity was investigated by DPPH assay. The yield of the essential oil was higher after the pretreatment of the leaves. The distillation method affected the quantitative composition of essential oil, but not its qualitative composition. The essential oil from the pretreated leaves had a higher content of camphor, verbenone and borneol (33.3%, 12.2% and 8.3%, respectively) and better antioxidant activity than the oil obtained by classical hydrodistillation. The advantage of the pretreatment method is the effective improvement of the extraction efficiency of rosemary essential oil from rosemary leaves.