  We reanalyze the annual modulation data observed by CoGeNT experiment and
show that if the annually modulated anomaly detected by CoGeNT is induced by
collision between dark matter particle and nucleus, it can be fitted by a $Z'$
model with inelastic dark matter and isospin-violating interaction, and the
constraint from XENON100 can be avoided. This $Z'$ model is strongly
constrained by collider physics that the upper bound of the mass of $Z'$ is
around twice of the mass of dark matter.
