In the plant community investigation, the size of plot area can affect the results of the community composition and biodiversity. Likewise, this problem might also exist in underground ecosystem. So far, little information is available in the comparison and exploration of the effects of soil sample dosages on the results of soil nematode community characteristics. Here, upland and paddy soil were selected, which are two typical cultivated soils in the red soil regions, to determine the gradients of soil amounts (10, 30, 50, 100, 150, 200, 300 and 500 g) on the nematode community based on the modified Baermann funnel method, with each level of soil dosage corresponding to different soil thickness from 0.70 cm to 3.51 cm. Results showed that no significant difference for nematode abundance per unit soil was observed among the soil dosage ranging from 10 g to 200 g (0.70 cm to 1.51 cm), however, soil exceeded 200 g (1.51 cm) significantly increased soil thickness and decreased nematode abundance per unit soil (P < 0.05). The genera number of nematode increased with increasing soil dosage. There was no significant difference for the proportion of nematode trophic groups when soil dosage was between 10 g and 200 g (0.70 cm to 1.51 cm). Nematode ecological indices such as richness, maturity, enrichment and structure indices were similar when soil dosage was from 30 g to 500 g (0.72 cm to 3.51 cm). Soil thickness significantly increased when soil exceeded 150 g. Together, it suggested that the optimal soil sample dosage for nematode separation in nematode community analysis should be from 50 g to 150 g or increase soil dosage under the condition of keeping soil thickness less than 1.00 cm.