Electron energy loss spectra (EELS) has been measured under reflection high-energy electron diffraction (RHEED) condition using a newly developed energy filtered RHEED measurement system to investigate incident and exit angle dependence of plasmon excitation process. As a specimen, clean Si(1 1 1)7 x 7 surface is used. From Poisson distribution analysis of EELS spectra, mean number of surface (n(s)) and volume (n(v)) plasmon excitation has been obtained. Dependency of n(s) on incident and exit angle of electron is different from that expected from Lucas theory. It is considered that refraction of electrons near surface region caused the difference. While nv value stays constant for glancing angle, which shows little dependence of effective penetration depth for volume plasmon excitation on glancing angle. (C) 2003 Elsevier Science B.V. All rights reserved.