Environmental water is a complexmatrix that contains innumerable natural nanoparticles of different sizes,compositions and shapes. The microfiltration with greater than 0.1 μm pore sizes was widely used to remove large colloids to prevent nebulizer jamming in the ICP-MS analysis. A series filter membraneswith different pore sizeswere used to investigate the influence of filtration on the determination of nanoparticle's size distribution at low number concentration. It could indicate whether filtration is a proper way to detect nanoparticle in the real environmental waters with SP-ICPMS. The samples of 10 ng/L 50 nm AgNPs and 12.5 ng/L 60 nm AuNPs were filtered through 0.22,0.45, 1, 5 μm membranes respectively and analyzed immediately using SP-ICPMS. The raw signal intensity of 10 ng/L 50 nm AgNPs and 12.5 ng/L 60 nm AuNPs obtained as a function of time by SP-ICPMS is illustrated in Fig. 6./mL.