Soil dielectric constant is basic for soil water content and soil froze-thaw status monitoring by microwave remote sensing, and also provides underlying boundary condition for plant and snow. However, present research on soil dielectric constant is lack of systematically measurement on high organic matter soils. This research classified soil natural organic matter substance into two types: humus and plant residue. With rigorous controlled-condition experiment, five black soils in the Northeast China with varied organic matter content and the meadow soil in Biandukou mixed with different content of poplar sawdust were measured to study the dielectric effect of humus and plant residue respectively. Results show that humus can decrease the bulk density of dry soil and decrease soil dielectric constant acting as an indirect mechanism. For wet soil under the same bulk density, the dielectric constant of soil with higher humus content is inclined to be larger. Comparison with Dobson model simulation shows that the impact of humus is around ±2 for real part and can reach 1 for imaginary part for wet soil with 25% gravimetric water content under a room temperature of 29℃. Compared with humus, plant residue influences both dry soil and wet soil significantly. Plant residue can effectively loose soil structure and bring in the dielectric property of a plant component. With gravimetric water content of 30%, the dielectric constant of Biandukou soil mixed with 20% poplar content is 4—7 lower for real part and 1—3 lower for imaginary part than pure Biandukou soil in average. Therefore, the natural organic substance in soil can change soil dielectric constant, and show its impact on microwave remote sensing.