Abstract: Chemical compositions of hydro-distilled Myrtus communis and Origanum vulgare leaves essential oils were examined by gas chromatography-mass spectrometry (GC/MS). Myrtus communis oil was found to contain thirty-one compounds, the major components being α-Pinene (26.99%) and 1,8-Cineol (20.37%), while O. vulgare oil analysis reached fifty-five compounds, the major components being thymol (25.48%) and carvacrol (10.29%). The 2,2-diphenyl-1-picrylhydrazyl (DPPH) free radical scavenging activities of M. communis and O. vulgare essential oils at various concentrations ranging from 25-200 μg mL–1 were determined. The 50% inhibition concentration (IC50) of O. vulgare essential oil (81.99 μg mL–1) was higher than Myrtus communis essential oil (122.78 μg mL–1). Human promyelocytic leukemia cell lines (HL-60, NB4) and Ehrlish Ascites Carcinoma Cells (EACC) were incubated in vitro with different levels of two essential oils ranged from 25-200 μg mL–1 for viability test and the percentages of dead cells were determined. The 50% Lethal Concentration (LC50) values indicated that the anticancer activity of O. vulgare essential oil was higher than M. communis essential oil against HL-60 and NB4 cell lines while The LC50 values indicated that the anticancer activity of M. communis essential oil was higher than O. vulgare essential oil against EACC. In in vivo assay, pre-initiation treatments with the both essential oils were more effective than initiation and post-initiation treatments, respectively on the tumor (EACC) transplanted female mice (decrease total EACC number and increase dead cells). Biological effects of both essential oils on normal mice indicated that all the obtained values in all experimental animals were within the normal range.