Most patients with gastrointestinal cancers refer to the health centers at advanced stages of the disease and conventional treatments are not significantly effective for these patients. Therefore, using modern therapeutic approaches with lower toxicity bring higher chance for successful treatment and reduced adverse effects in such patients. The aim of this study is to evaluate the effect of avocado fruit extracts on inhibition of the growth of cancer cells in comparison with normal cells. In an experimental study, ethanol, chloroform, ethyl acetate, and petroleum extracts of avocado (Persea americana) fruit were prepared. Then, the effects if the extracts on the growth of esophageal squamous cell carcinoma and colon adenocarcinoma cell lines were evaluated in comparison with the control group using the MTT test in the cell culture medium. Effects of the four extracts of avocado fruit on three cells lines of peripheral blood mononuclear cells, esophageal squamous cell carcinoma, and colon adenocarcinoma were tested. The results showed that avocado fruit extract is effective in inhibition of cancer cell growth in comparison with normal cells (P<0.05). Avocado fruit is rich in phytochemicals, which play an important role in inhibition of growth of cancer cells. The current study for the first time demonstrates the anti-cancer effect of avocado fruit extracts on two cancers common in Iran. Therefore, it is suggested that the fruit extracts can be considered as appropriate complementary treatments in treatment of esophageal and colon cancers.