C. citratus essential oil and carvacrol have shown an antitumor effect on breast tumor cell lines; the main objective of this research was to evaluate the antitumor effect of the essential oil of Cymbopogon citratus (EOCc) and carvacrol on 7,12-dimethylbenz [a] anthracene (DMBA)-induced breast cancer in female rats. Cancer was induced by a single administration of DMBA at dose of 80 mg/kg body weight (BW). A total of 54 female Holtzman rats were randomly assigned into 9 groups (n = 6). Group I: PS (Physiological saline); Group II: DMBA; Groups III, IV, and V: DMBA + EOCc at doses of 50, 100 and 200 mg/kg/day BW, respectively; Groups VI, VII, and VIII: DMBA + carvacrol at doses of 50, 100 and 200 mg/kg/day BW, respectively; and group IX: DMBA + EOCc + carvacrol at doses of 100 mg/kg/day BW. The treatment lasted 14 weeks. As results, EOCc showed a reduction in tumors as well as necrosis and mitosis. Animals treated with carvacrol did not show necrosis, mitosis, or infiltration. Carvacrol at dose of 100 mg/kg/day BW revealed a significant decrease in the cumulative tumor volume down to 0.11 ± 0.05 cm3 compared to 0.38 ± 0.04 cm3 of the DMBA group (p < 0.01). It is concluded that EOCc and carvacrol had an antitumor effect on DMBA-induced breast cancer in female rats.