Greater kernel processing increases starch digestibility in whole-plant corn silage (WPCS). Corn silage processing score (CSPS), the percentage of starch passing through a 4.75 mm sieve, is widely used to assess degree of kernel breakage in WPCS. However, the geometric mean particle size (GMPS) of the kernel fraction that passes the 4.75 mm sieve has not been well described. Therefore, the objective of this study was to evaluate the relationship between CSPS and GMPS of the kernel fraction. Samples of WPCS (n = 80) from 3 field trials representing varied theoretical lenght of cut settings and processor types and settings were evaluated. Each sample was divided in 2 and then dried at 60°C for 48 h. The CSPS was determined in duplicate on one of the split samples, while on the other split sample the kernel and stover fractions were separated using a hydrodynamic separation procedure. After separation, the kernel fraction was re-dried at 60°C for 48 h in a forced-air oven and dry sieved to determine GMPS (µm) and surface area using 9 sieves with nominal square apertures of 9.5, 6.7, 4.75, 3.35, 2.36, 1.70, 1.18, 0.589 mm and pan. Regressions to determine linear and quadratic relationships were performed using Proc Reg of SAS (SAS Institute, 2004). Linear relationships between CSPS and kernel fraction GMPS (R² = 0.11; P = 0.01), surface area (R² = 0.06; P = 0.03) and proportion passing through the 4.75 mm screen (R² = 0.34; P = 0.001) were poor. Strong quadratic relationships between proportion of kernel fraction passing through the 4.75 mm screen and kernel fraction GMPS (R² = 0.70; P = 0.001) and surface area (R² = 0.56; P = 0.001) were observed. Corn silage processing score was poorly correlated with kernel fraction GMPS and surface area. These findings suggest that hydrodynamic separation and dry sieving of kernel fraction may provide better assessment of kernel breakage in WPCS.