  We perform a calculation of Sivers function in a light-cone SU(6)
quark-diquark model with both scalar diquark and vector diquark spectators. We
derive the transverse momentum dependent light-cone wave function of the proton
by taking into account the Melosh-Wigner rotation. By adopting one-gluon
exchange, we obtain a non-vanishing Sivers function of $down$ quark from
interference of proton spin amplitudes. We analyze the $\frac{|P_{h\perp}|}{M}$
weighted Sivers asymmetries in $\pi^+$, $\pi^-$ and $\pi^0$ electroproduction
off transverse polarized proton target, averaged and not averaged by the
kinematics of HERMES experiment.
