  We study the production of heavy sterile neutrino $N$, $e^+ e^- \to N
\nu({\bar \nu})$, at the Circular Electron Positron Collider(CEPC) and its
$ljj$ signal in its decay to three charged fermions. We study background events
for this process which are mainly events coming from W pair production. We
study the production of a single heavy sterile neutrino and the sensitivity of
CEPC to the mixing of sterile neutrino with active neutrinos. We study the
production of two degenerate heavy sterile neutrinos in a low energy see-saw
model by taking into account the constraints on mixings of sterile neutrinos
from the neutrino-less double $\beta$ decay experiment and the masses and
mixings of active neutrinos. We show that CEPC under proposal has a good
sensitivity to the mixing of sterile neutrinos with active neutrinos for a mass
of sterile neutrino around 100 GeV.
