In spite of their widespread use, toxicity of silica nanoparticles (SiO2 NPs) to mammalian has not been extensively investigated. In the present study, it is aimed to investigate the effects and the mechanism of action of 20 nm sized SiO2 NPs on isolated uterine smooth muscle. A total number of 84 preparations of uterine strips were used in the experiments. Study was designed as four groups: group I (control), group II (0.2 mM SiO2 NPs), group III (0.4 mM SiO2 NPs) and group IV (0.8 mM SiO2 NPs). Spontaneous contractions were recorded using mechanical activity recording system. Superoxide dismutase (SOD) and glutathione peroxidase (GSH-Px) activities and malondialdehyde (MDA) levels were measured using the spectrophotometric methods. Apoptosis of the cells was detected using immunofluorescence staining assay. SiO2 NP distribution and ultrastructural changes were determined by transmission electron microscopy. In groups II-IV, the frequency of contraction was significantly lower than that of the group I, whereas the contraction energy significantly decreased only in group IV. SOD and GSH-Px activities were significantly lower in experimental groups compared to the control group. MDA level and apoptotic cells were significantly higher in all SiO2 groups compared to the control group. Numerous SiO2 NPs in cytoplasm and connective tissue were observed in all dose groups. These findings showed that 20 nm sized SiO2 NPs enter the connective tissue and cytoplasm of uterine muscle cells and cause oxidative stress and apoptosis leading to impaired uterine contractile activity.