The cytotoxic effects of silica nanoparticles (SNPs) on different human cancer cells, as well as the uptake kinetics and pathways of SNPs have been studied here. SNPs with the diameter of approximate to 20 nm induced a dose-dependent cytotoxicity in both gastric cancer cells (MGC80-3) and cervical adenocarcinoma epithelial cells (HeLa), but MGC80-3 cells were more susceptible to the cytotoxic effect induced by SNPs. Changes in the nuclear morphology and flow cytometric analysis with annexin V/PI double staining show that SNPs induced a higher degree of apoptosis in MGC80-3 cells. Accordingly, more remarkable reactive oxygen species (ROS) burst is detected in SNP-treated MGC80-3 cells. Using fluorescein isothiocyanate (FITC)-labeled SNPs and flow cytometry, it is found that the uptake of SNPs is more efficient in MGC80-3 than in HeLa cells. SNPs are internalized into both cancer cells through energy-dependent pathway. Inhibitor studies with dynasore and methyl-beta-cyclodextrin show that these cancer cells took up 20 nm SNPs mainly through the caveolin-mediated endocytosis, while in HeLa cells SNPs internalization was also via dynamin-dependent clathrin-mediated pathway. These findings indicate that SNPs cause differential cytotoxic effects in different human cancer cells, which might be related to the uptake process and efficiency toward these cancer cells.