The laboratory incubation experiments were conducted to study the toxicity of silver nanoparticles with different particle sizes on Nitrosomonas europaea, and the effects of two sizes of nanosilver (10nm and 50nm) on the bacterial growth, nitrogen transformation, cellular structure, reactive oxygen generation and gene expression were investigated. The results showed that nanosilver inhibited the growth of N. europaea. With the extension of exposure time, the inhibition rate of bacterial growth activity increased and reached to maximum at 4h. In the medium, the transformation rate of NH₄⁺ to NO₂^-was slowed down, and the nitrogen transformation ability by N. europaea was reduced. Scanning electron microscopy (SEM) images showed that nanosilver heavily damaged the cell membrane by causing holes on the surface of bacteria. Transmission electron microscope (TEM) images showed that the nuclear material inside the bacteria was disappeared and the boundary of the cytoplasmic membrane was blurred. Flow cytometry (FCM) was employed to detect that nanosilver could generate intracellular reactive oxygen (ROS) in the cells. qRT-PCR technology was used to determine the expression of amoA, hao and merA of N. europaea after the exposure to nanosilver, and it was found that nanosilver inhibited the expression of the functional genes. In conclusion, nanosilver could interact with cell membrane, generated oxidative stress damage and inhibited the expression of functional genes amoA and merA of N. europaea, which further affected the transformation process of ammonium nitrogen. In addition, the toxicity of nanosilver with small particle size was stronger than that of large particle size.