<p>We examined the spatial variability for the characteristics of surface soil layers in forested headwater catchments, which is important for estimating soil erodibility (K-factor). Soil samples with 30&#xa0;cm in depth from surface were collected at 26 locations within two neighboring headwater catchments, located in Tanzawa Mountains in Japan. Soil bulk density, particle size and organic matter contents for calculating soil erodibility factor was analyzed 5&#xa0;cm interval with depth. Soil bulk density ranged from 0.4 to 1.1&#xa0;g/cm³ (mean: 0.7; SD: 0.2) with low bulk density associated with litter ground cover condition and 30⁰ of topography. Soil organic matter contents ranged from 5 to 12% (mean: 9; SD: 2) with high values occurred at ridgeline due to litter ground cover condition. Estimated soil erodibility ranged at 5&#xa0;cm depth was 0.011 to 0.018 Mg h MJ⁻¹ mm⁻¹, while K factor in 30&#xa0;cm depth was 0.012 to 0.017 Mg h MJ⁻¹ mm⁻¹. Because of the calculation for the soil erodibility mainly depend on the particle size, K factor based on 30&#xa0;cm soil depth was from 0.012 to 0.017 Mg h MJ⁻¹ mm⁻¹ with 0.014 Mg h MJ⁻¹ mm⁻¹ of mean value (SD: 0.002). Because of soil particle size was not spatial varied, K-factor did not significantly differ depending on the locations with suspect to slope gradient, aspects, and altitude within catchments. Thus, mean soil erodibility can be applicable for estimating soil erosion using RUSLE model. These findings support the simplification of erosion risk assessments by allowing the use of representative K-factor values in forested headwater catchments, potentially improving the efficiency and accuracy of erosion modeling in mountainous regions.</p>