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 .