An essential oil (EO) is defined internationally as the product obtained by hydrodistillation, of a plant or some parts of it. They are aromatic oily liquids, volatile, characterized by a strong odour, rarely coloured, and generally with a lower density than that of water. They can be synthesized by all plant organs (flowers, buds, seeds, leaves, twigs, bark, herbs, wood, fruits and roots) and therefore extracted from these parts, where they are stored in secretory cells, cavities, ducts, epidermal cells, osmophores or glandular trichomes. EOs have a complex composition. The great majority of components identified in EOs include monoterpenes and sesquiterpenes. Nevertheless, phenylpropanoids are also important components of some essential oils. EOs extracted from Thymbra capitata and Helichrysum italicum collected in Algarve (Portugal) were investigated for their antitumor activity, specifically looking at effect on viability of human colorectal cancer (RKO) and breast cancer (MCF7) cells using MTS assay. The serial doubling dilution of the essential oil was prepared in DMSO. Both EOs tested, at final concentration 1 µL/mL and dilution 1:10, lead to a decrease in cell viability for both RKO (colorectal) and MCF7 (breast) cancer cell lines. Specifically, average IC50 values were 2.82E-03 µL/mL and 1.08E-02 µL/mL for T. capitata and 1.03E-01 µL/mL and 1.30E+01 µL/mL for H. italicum (IC50s for RKO and MCF7 cell lines, respectively). It has long been recognised that some EOs have antimicrobial, antibacterial, antioxidant, antiviral, antimycotic, antiparasitic, insecticidal activities. However, studies into their anti-tumoral activities are scarce. This study, indicates that Thymbra capitata and Helichrysum italicum EOs exhibited antitumor activity against RKO and MCF7 cells, deserving further research into both the mechanism of the observed anti-tumour activity and potential cancer therapeutic applications.