We evaluated the estimation of the leaf area index (LAI) and the fraction of absorbed photosynthetically active radiation (FAPAR) of green plant canopies from top-of-the-canopy (TOC) spectral indices by using the PROSAIL model under the possible constraints of leaf and soil spectra. For the LAI estimation, the ratios (B1-B3)/(B1+B3) and B1/B3 provided fewer estimation errors than (B4-B3)/(B4+B3) or B4/B3 when the variation in the soil spectral reflectance was small or LAI was large. Here, B1, B3 and B4 denote the blue, red and near-infrared bands of Landsat ETM+, respectively. For the FAPAR estimation, the ratios (B5 -B7)/(B5+B7) and B5/B7 provided fewer estimation errors than (NIR-R)/(NIR +R) or NIR/R for a FAPAR value of 0.3-0.7 when the variation in the soil spectral reflectance was large. Here, B5 and B7 denote the bands with wavelengths 1.55-1.75 and 2.09-2.35 mu m, respectively. These were maintained for various conditions of the solar incident zenith angle (theta(s)), leaf angle distribution (LAD), canopy hotspot parameter (s(1)) and clumping index (Omega).