Breast cancer is the most common disease in women worldwide and it is the most common cancer in the Arab world. It affects women at an early age compared with women in western countries. Doxorubicin (DOX) is a chemotherapeutic drug and is highly effective in advanced breast cancer. Unfortunately, the cytotoxic activity of DOX affects normal cells as well as cancer cells, which leads to severe heart damage. Many strategies have been investigated to protect the heart against DOX-induced cardiotoxicity one of them is the search for a natural compound with chemopreventive or anticancer properties that can be used with DOX. Recent studies proved the effect of the natural product astaxanthin (AST) as an anti-cancer. Therefore, the present study aimed to study the effect of AST co-treatment with DOX on the growth of the MCF-7 breast cancer cell line and investigate the possible mechanism to overcome the side effects and multidrug resistance of DOX treatment. To assess these effects, we investigated the DOX effect on apoptosis induction, cell cycle phase distribution, cellular uptake, ABCB1mRNA expression and p-glycoprotein (P-gp) activity in breast cancer cells in the presence and absence of AST. The addition of AST (133 and 250µg/ml) increased DOX cytotoxicity which manifested as a significant decrease in IC₅₀ compared to the cells treated with DOX alone. Moreover, flow cytometric analysis of the MCF-7 cells treated with DOX (4.5μg/ml) simultaneously with AST (250μg/ml) showed the enhanced arrest of the cells in G₀\G₁ (35.50 %). In addition, apoptotic phase cells were significantly increased to 39.13% when AST (250μg/ml) was combined with DOX compared to 27.3% in DOX alone.  Also, AST (133 µg/ml and 250 µg/ml) significantly increased the cellular uptake of DOX as compared to DOX alone.  The addition of AST (133 and 250μg/ml) combined with DOX resulted in a concentration-dependent decrease and almost complete reversal of DOX-induced increase in ABCB1 mRNA expression to the control value. The present study concluded that AST chemosensitizers DOX cytotoxicity against the proliferation of MCF-7 human breast cancer cells. This could be explained by induction of apoptosis, enhanced DOX cellular uptake, downregulation of ABCB1 gene expression and inhibition of P-gp activity.