It is aimed to determine the effects of naphthoquinones as juglone and curcumin application on cell viability and expression analyzes of CYP3A4 and MTATP6 genes in MCF-7 and MDA-MB-231 human breast cancer cell lines. MCF-7 and MDA-MB-231 cells were incubated, were replaced with containing various concentrations of 5, 10, 15 μM curcumin and 5, 10, 15 μM juglone for MCF-7 and 1, 5, 10 μM curcumin and 1, 2, 3 μM juglone for MDA-MB-231 for 24 h. CYP3A4 and MTATP6 gene expression levels in both cell lines were determined by quantitative real-time polymerase chain reaction (qPCR) method and western blot method. IC<inf>50</inf> values for 24 h were found as 22.41 μM for curcumin, and 16.27 μM for juglone in MCF-7, and 10.43 μM for curcumin, and 3.42 μM for juglone in MDA-MB-231 cells. Curcumin showed anti-proliferative, and antioxidant effects. CYP3A4 and MTATP6 gene expressions were decreased in MCF-7 breast cancer cell line when the cells treated with juglone or curcumin. CYP3A4 and MTATP6 gene expressions were decreased at all application doses of juglone in MDA-MB-231 cells whereas CYP3A4 and MTATP6 protein levels were only decreased at 10 μM curcumin compared with the control group.