  The associated production of $Z$ boson and a pair of new quarks at the Large
Hadron Collider (LHC) is studied. The cross sections for both sequential
fermions and vector-like fermions are presented. It is found that for
sequential fermions the cross sections can reach $1\sim 10^2$ fb for heavy
quark mass $m_Q$ from 1000 GeV to 200 GeV.
  For vector-like quarks, the cross sections are suppressed by mixing parameter
$\sin\theta_L$. Focusing on process $pp \to b'b'$, we investigate the
possibility of detecting the $6l+2j$ signal. For a $b'$ with light mass and a
large branching ratio of $b'\to bZ$, it is found that only several signal
events (parton level) can be produced with 1000 fb$^{-1}$ integrated
luminosity. Although the signal events are rare, all the final states are
produced centrally and multi lepton final states are clear at hadron collider,
which could be easily detected.
