  The COSINE-100 dark matter search experiment has started taking physics data
with the goal of performing an independent measurement of the annual modulation
signal observed by DAMA/LIBRA. A muon detector was constructed by using plastic
scintillator panels in the outermost layer of the shield surrounding the
COSINE-100 detector. It is used to detect cosmic ray muons in order to
understand the impact of the muon annual modulation on dark matter analysis.
Assembly and initial performance test of each module have been performed at a
ground laboratory. The installation of the detector in Yangyang Underground
Laboratory (Y2L) was completed in the summer of 2016. Using three months of
data, the muon underground flux was measured to be 328 $\pm$ 1(stat.)$\pm$
10(syst.) muons/m$^2$/day. In this report, the assembly of the muon detector
and the results from the analysis are presented.
