Inflammation is a common mechanism of the immune system that protects the host body from injuries caused by physical wounds and non-self-invaders, while chronic inflammation might cause tissue damages and is associated with many diseases such as rheumatoid arthritis, Alzheimer’s disease, and various physical dysfunctions. The non-steroidal anti-inflammatory drugs (NSAIDs) are widely used because of cyclooxygenase-2 (COX-2) inhibitory function. However, the drugs can also act on COX-1 leading to an unfavorable condition such as irritation of the stomach and increase the risk of bleeding. The aim of this study was to evaluate anti-inflammatory effects of 11 Thai herb essential oils on lipopolysaccharide (LPS)-induced inflammation in phorbol 12-myristate 13-acetate (PMA) differentiated human monocyte cell-line (THP-1) cells., The COX-2 activity and level of prostaglandin E2 (PGE2) were examined. The results showed that among 11 Thai herb essential oils, lesser galanga essential oils exhibited the significant COX-2 and PGE2 inhibitory activities. β-ocimene, the major compound of lesser galanga essential oil, was further evaluated for the anti-inflammatory effects. It was clearly demonstrated that β-ocimene effectively inhibited COX-2 activity and lowered PGE2 level in a dose-dependent manner with IC50 of 75.64 and less than 20 µg/mL, respectively. It was noteworthy that lesser galanga essential oil possessed higher inhibitory effect on COX-2 activity and PGE2 levels than its major compound, β-ocimene. In conclusion, essential oil of lesser galanga and its major compound could be potentially developed as the anti-inflammatory agents.