Chemical investigation of the ethanol extract of the stalks and infructescence of Sibiraea leavigata led to the isolation of two new monoterpenes named (4R)-2-(2-hydroxy-4-methyl-3-pentenyl)furan-2(5H)-one (1) and (2R,4R)-2-(2-hydroxyethyl)-4-(2-methyl-1-propenyl)furan-5H-2-one (2) along with eight known phenylpropanoids (3–10). Their structures were established on the basis of the interpretation of spectroscopic data and electronic circular dichroism (ECD) calculations. In addition, all of these isolates were evaluated for their cytotoxic activity. The results showed that compound 3 displayed moderate cytotoxicity with IC₅₀ values ranging from 10.8 to 49.2 μg mL⁻¹ against five cell lines. While 1 showed selective promotion effects on proliferation of gastric cancer MGC803 and RSC96 cell lines.