Objective:To investigate the effects of 5 nm gold nanoparticles(GNPs) on newborn rat cortical neurons and the underlying mechanism. Methods:GNPs were prepared by the NaBH4 reduction method. Transmission electron microscope(TEM) and ultraviolet-visible(UV-Vis) spectra were obtained to characterize GNPs. Immunofluorescence staining was performed to indentify primary neurons and calculate the purity. After culture for 72 h, primary cortical neurons were incubated with neuron culture medium with(600, 1 200, 2 400 μg/L) or without GNPs for 48 h. TEM was used to investigate the celluar uptake and distribution of GNPs; Cell counting kit-8(CCK-8) was employed to evaluate cell viability; TUNEL assay was performed to analyze apoptosis; MDA and SOD were measured to assess oxidative stress. Results:TEM and UV-Vis showed that GNPs were homogeneous in size and shape and well dispersed in culture medium. The purity of primary neurons was(82.7 ± 2.3)%. GNPs were mainly distributed in cytoplasm, lysosomes and vesicles. The cell viability was found to decrease with increasing concentrations of GNPs(P < 0.05). In addition, GNPs were found to induce apoptosis in neurons. Particularly, higher concentrations(1 200 μg/L and 2 400 μg/L) markedly increased neural apoptosis(P < 0.05). Furthermore, our results suggested pro-oxidant ability of 5 nm GNPs,which is supported by escalating MDA and reducing SOD along with increasing concentrations of GNPs(P < 0.05). Conclusion:Five nm GNPs can significantly reduce cell viability and induce apoposis in newborn rat cortical neurons, which potentially links to oxidative stress.