Electron energy-loss spectra ofα-rhombohedral boron (α-r-B) were obtained from perfect crystalline areas of 180 nm diameter. The onset of the spectral intensity was observed at 2.4 eV (band gap energy). The volume plasmon peak due to thespvalence electrons is at 24.5 eV with a full width at half maximum of 7.5 eV. The real partε1(ω) and imaginary partε2(ω) of the dielectric function were obtained from the loss-function by Kramers-Kronig analysis. The value ofε1(0) was 6.5.ε2(ω) showed clear peaks, not observed inε2(ω) ofβ-rhombohedral boron (β-r-B) and amorphous boron (am-B). K-shell excitation spectra show several peaks, which are not observed in the spectra ofβ-r-B and am-B. The characteristic structure of the peaks showed good agreement with a recentab initiocalculation within the local density approximation.