Background: Essential oils are the volatile fraction of aromatic and medicinal plants created after extraction by steam or water distillation. Species of the genus Citrus (Rutaceae) have been widely used in traditional medicine as volatile oils and are currently the subject of numerous research. Citrus essential oil consists of different terpens that have antitumor activities. This study determines the cytotoxic effect of the essential oils of Citrus limon L. peels on a colorectal cancer cell line (LIM1863).Methods:We harvested four samples from four locations in Syria. Essential oils were prepared by hydrodistillation and analyzed by Gas chromatography-mass spectrometry (GC-MS).Various concentrations ofessential oils (0.5-48 µg/ml) were added to cultured cells and incubated for 72 h. Cell viability was evaluated byMTT-based cytotoxicity assay.Results:We noted 18 components that represented 98.81% of the total oil content. The major components were: limonene (61.8%-73.8%), γ-terpinene (9.4%-10.4%), β- pinene (3.7%-6.9%), O-cymene (1%-2.4%),and citral (0.8%-5.4%).The obtained IC50 value range of Citrus limon essential oils was 5.75-7.92 µg/ml against LIM1863.Conclusion: This study revealed that Syrian Citrus limon essential oil has a cytotoxic effect on the human colorectalcarcinoma cell line LIM1863 when studied in vitro.