Abstracr:The energy spectrum of cosmic rays around the
knee measured by Tibet air-shower experiment is summarized
and its characteristic features are discussed under two
possible scenarios. The result of Tibet experiment measured
over wide range of 1014.1017 eV with high statistics provided
details of the knee at the energy around 4¡ß1015 eV.
The study of the chemical composition based on measurements
of proton and helium spectra obtained from airshower
core detection indicates the dominance of heavy nuclei
around the knee. Such feature can be explained either
by contribution of nearby sources with source composition
dominated by heavy nuclei or by nonlinear effect in diffusive
shock acceleration mechanism.