Graphene magnetite nanocomposites (G/FeO) exhibit light photothermal conversion upon enhancement by 808 nm IR laser excitation. We evaluated the cytotoxic and photothermal effects of G/FeO on a HepG2 human liver cancer cell model. Graphene nanosheets (rGO), magnetite nanoparticles (FeO), and G/FeO were prepared by chemical methods and characterized using transmission electron microscopy, Raman spectroscopy, zeta analysis, and vibrating sample magnemeter. Dark and light cytotoxicity were screened with colorimetric Sulforhodamine B cell viability assay after 24 and 48 hours. DNA fragmentation and some apoptotic genes on a transcriptional RNA level expression were performed. All prepared nanomaterials were evaluated for their photothermal effect at concentrations of 10 and 50 µg/mL. The power density incident on the cells by 300 mW 808 IR diode laser was 0.597 W/cm. Treatment of HepG2 with 400 µg/mL of rGO, FeO, and G/FeO showed alteration in cell morphology after 24 hours of cell treatment and revealed toxic effects on cellular DNA. Evaluation of the cytotoxic effects showed messenger RNA (mRNA) in and apoptotic genes, but no expression of mRNA of after 24 hours of cell exposure, suggesting the involvement of an intrinsic apoptotic caspase-independent pathway. A photothermal effect was observed for G/FeO after irradiation of the HepG2 cells. A marked decrease was found in cell viability when treated with 10 and 50 µg/mL G/FeO from 40% to 5% after 48 hours of cell treatment. Results indicate that G/FeO nanocomposite was effective at transformation of light into heat and is a promising candidate for cancer therapy.