It is a recognized view that particle size could play a significant role in catalytic performance. To obtain the nanoparticles with the different size, a series of bimetallic PtPd/SiO 2 −Al 2 O 3 catalysts subjected to hydrothermal treatment in different time were prepared and applied for NO oxidation under diesel exhaust. The experimental results confirmed that the average noble particle size was gradually increased at a different variation rate, along with aging time. An increasing of average particle size resulted in the reduction of the corresponding NO turnover frequencies (TOFs) by different degrees. Thus, a new sight on the relationship between particle size and NO catalytic activity is obtained. In the aging process, the overall trend of noble metal particles got larger along with receding NO catalytic performance, yet in a period, a few small particles presented more bridged/chelating nitrate and ionic nitrate species, further retarding its rate of loss activity.