Abstract: This study was conducted to investigate the effects of water volume, sample weight and sieving time on the mean particle size of pelleted diet, digesta and feces during wet-sieving, and to provide a reference for the determination of particle size for pelleted diet, digesta and feces of pig. The mean particle size was assayed with wet-sieving using a set of 6 sieves sized 2.0, 1.0, 0.5, 0.25, 0.106 and 0.072 mm. The water volume of 1,1.25 or 1.5 L, sample weight of 7.5 or 10 g for pelleted diet, 15 or 30 g for ileal digesta, 10 or 20 g for feces and sieving time of 4 or 5 min were used in a 3×2×2 factorial completely randomized arrangement. There were 12 treatments for each type of sample. The effect of wet-sieving conditions on the measurement of geometric mean particle size of samples was investigated. The results showed that:1) Increment of water volume significantly decreased the proportion of the mass content of the particles more than 1 mm in size(P<0.05), but significantly increased the proportion of the mass content of soluble matter in pelleted diet, ileal digesta and feces (P<0.05); Increment of the sample weight significantly increased the proportion of the mass content of the particles more than 1 mm in size(P<0.05), but significantly decreased the proportion of the mass content of soluble matter or particles less than 0.072 mm (P<0.05). However, there was no significant effect of sieving time on the proportion of the mass content on each sieves. 2) Water volume and sample weight extremely significantly affected the determination of mean particle size for pelleted diet, ileal digesta and feces (P<0.01), but significant effect of the sieving time was only observed on the determination of ileal digesta(P<0.05). The determined value of mean particle size was significantly higher in 1 L of water volume than 1.25 or 1.5 L (P<0.05). Moreover, particle size was significantly different between 1.25 and 1.5 L of water volume for feces (P<0.05), but there was no significant differences between 1.25 and 1.5 L of water volume for ileal digesta and pelleted diet. The sample weight of pelleted diet ranged from 7.5 to 10 g, ileal digesta from 15 to 30 g, and feces from 10 to 20 g led to the difference of less than 12 μm in determined particle size. In summary, in the determination of particle size for pelleted diet, digesta and feces of pig using wet-sieving method, the sample weight of pelleted diet ranged from 7.5 to 10 g, ileal digesta from 15 to 30 g, feces from 10 to 20 g, 1.25 L of water volume and 4 min of sieving time are the optimized condition to obtain a relatively stable result.