With recent development of nanotechnology, nanomaterials (NMs) have been developed with innovative function and expected to cause a paradigm shift in various industry such as cosmetics, medicine and food. NMs begin to establish firm position in Japan as base of various industrials, in fact, a part of them have been already applied to various products. On the other hand, it is suggested that these innovative properties may induce unknown biological responses. It is concerned about the effect of these innovative properties to human health. Based on these situations, to evaluate risk of NMs, it is started to collect information about safety of NMs (Nano Safety Science). With this in mind, we analyzed the relationship between particle size and the in vitro effect of amorphous nanosilica (nSP) using human keratinocyte cells (HaCaT). Our results indicate that exposure to nSP of 70 nm diameter (nSP70) induced an elevated level of reactive oxygen species (ROS), leading to DNA damage. On the other hand, a markedly reduced response was observed using submicron-sized silica particles. Next, we investigate relationship between endocytosis, generation of ROS and DNA damage using endocytosis inhibitor, cytochalasin D (CytoD). As result, CytoD -treatment reduced nSP70-mediated ROS generation and DNA damage. This suggested that endocytosis is involved in nSP70-mediated cellular effects. Thus, particle size affects amorphous silica-induced ROS generation and DNA damage in HaCaT cells. We believe that clarification of the endocytosis pathway of nSP will provide useful information for hazard identification as well as the design of safer forms of nSP.