Green herbage (GH) mass is an important vegetation characteristic of grazing pastures because it is closely related to the feed selection by animals and the nutritive quality of the diet. Zoysia-dominated pastures contain a large amount of dry herbage material, which makes it difficult to measure GH mass accurately. In this study, a portable hyperspectral radiometer was used to relate spectral characteristics (range = 350-1050 nm) to the GH mass in Zoysia-dominated pastures. The spectral reflectance attributes (R) of 485, 675 and 770 nm and the first differential reflectance attributes at 520, 570 and 720 nm had a positive or negative correlation to the GH mass. Of these, a combination of the spectral reflectance attributes at 485 and 770 nm (R-770/R-485) provided an effective vegetation index for monitoring GH mass. The regression model with this index is sufficient for the practical estimation of GH mass in Zoysia pastures (r = 0.83, evaluation index value = 21.4%).