The essential oil from the roots of Cyperus pilosus Vahl (Cyperaceae) was investigated. Sampling was carried out at various times (April, May, July, September, August, December 1977) and locations. The essential oil was extracted by distillation and analyzed using gas chromatography (GC) with a 3% OV-17 column, and spectroscopic techniques including mass spectrometry (MS), infrared spectroscopy (IR), nuclear magnetic resonance (NMR), and ultraviolet spectroscopy (UV). A new compound (peak 53) was identified as (2E,6E,10E)-3,7,11-trimethyl-2,6,10-dodecatrienedioate [1] based on spectral data (e.g., MS: m/z 294 [M]+, characteristic fragment m/z 114; IR: ν 1720 cm⁻¹ for α,β-unsaturated ester; UV: λ_max 219 nm; ¹H-NMR: δ 3.70 and 3.74 (3H each, s, methoxycarbonyl groups); ¹³C-NMR) and chemical reactions (saponification to yield [2], reduction with lithium aluminum hydride to [3], catalytic hydrogenation to [4]). The composition of the essential oil was presented in Table 2, with methyl (2E,6E)-farnesate being a major component.