  More than a century after the performance of the oil drop experiment, the
possible existence of fractionally charged particles FCP still remains
unsettled. The search for FCPs is crucial for some extensions of the Standard
Model in particle physics. Most of the previously conducted searches for FCPs
in cosmic rays were based on experiments underground or at high altitudes.
However, there have been few searches for FCPs in cosmic rays carried out in
orbit other than AMS-01 flown by a space shuttle and BESS by a balloon at the
top of the atmosphere. In this study, we conduct an FCP search in space based
on on-orbit data obtained using the DArk Matter Particle Explorer (DAMPE)
satellite over a period of five years. Unlike underground experiments, which
require an FCP energy of the order of hundreds of GeV, our FCP search starts at
only a few GeV. An upper limit of $6.2\times 10^{-10}~~\mathrm{cm^{-2}sr^{-1}
s^{-1}}$ is obtained for the flux. Our results demonstrate that DAMPE exhibits
higher sensitivity than experiments of similar types by three orders of
magnitude that more stringently restricts the conditions for the existence of
FCP in primary cosmic rays.
