Photodynamic therapy (PDT) is a technique that can be used as a complementary therapy in cancer treatment combined with other therapeutic modalities. Quercetin (QCT) is known to be effective in the treatment of cancer, by reducing the cell viability of different cancer cell lines. This study aimed to evaluate the influence of different concentrations of QCT in PDT on the viability, mitochondrial membrane potential and induction of apoptosis/necrosis in the human larynx carcinoma cells (HEp-2). The HEp-2 cells were treated with aluminum phthalocyanine tetrasulfonate (AlPcS4) and QCT and subsequently irradiated with a diode laser light (685 nm, 35 mW, 4.5 J/cm2). The results demonstrated that treatment of HEp-2 cells with high concentrations of QCT (at least 50 μM) reduced cell viability. This response was enhanced in cells subjected to PDT supplemented with high concentrations of QCT. In addition, was observed decrease in the mitochondrial membrane potential and characteristics of late apoptosis and/or initial necrosis process. QCT at concentrations from 50 μM improves PDT-induced cytotoxicity by significantly reducing cell viability by apoptosis and/or necrosis, and mitochondrial membrane potential of Hep-2 cells.