  An anomaly-free $U(1)'$ effective Lagrangian as the most common new physics
beyond the standard model is proposed to survey the maximal parameter space
constrained by electroweak precise measurements at the LEP and direct detection
in dilepton decay channel at the LHC at $\sqrt{s}=7$ TeV. By the global fit of
effective couplings of $Z$ boson to the SM fermions,
$\Delta_{11},\Delta_{21},g_2\Delta_{31}$ related to mixings and $r$ related to
$U(1)'$ charge assignment are bounded. The allowed areas are plotted in the not
only $r$-$g_2$ but also $m_{Z'}$-$g_2$ planes, which show that a sub-TeV $Z'$
is still permissible as long as the coupling $g_2\sim 0.01$. The results
provides a prime requirement to an extra $U(1)'$ gauge boson and hinds the
direction of possible new physics beyond the standard model. The possible
signal in dilepton decay channel at LHC at $\sqrt{s}=14$ TeV is also provided.
