  The energy dependence of the radii size of jets are studied in detail by the
HBT correlation method using Monte Carlo Simulation generator Jetset7.4 to
produce 40,000,000 events of e$^+$e$^-$ collisions at $\sqrt s =30$, 50, 70,
91.2, 110, 130, 150 and 170 GeV. The radii of jets are measured using the HBT
correlation method with the indistinguishability of identical final state
pions. It is found that the average radii of quark-jets and gluon-jets are
independent of the c.m. energy of e$^+$e$^-$ collisions. The average radius of
quark-jets are obviously larger than that of gluon-jets. The invariable average
radii of quark-jets and gluon-jets in e$^+$e$^-$ collisions are obtained at the
end of parton evolvement.
