The volatile oil from Lysimachia trientaloides Hemsl. was obtained by steam distillation. The chemical constituents were separated and identified by gas chromatography-mass spectrometry (GC-MS). The relative contents in the volatile oil were determined by peak area normalization. The yield of oil by steam distillation was 0.11%, and 40 chemical constituents were separated and identified. Terpenic series and their oxo-derivatives are major chemical constituents in the oil. The main compounds were patchouli alcohol(22.54%), L-bornyl acetate(16.17%), gamma-gurjunene(3.27%), delta-guaiene(2.62%), nerolidol(2.02%), linalool(1.99%) and palmitic acid(1.96%).