The goal of the COSINE-100 experiment is to confirm or refute the controversial observation of a WIMP Dark Matter like annual modulation by the DAMA/LIBRA experiment. Measurement of neutron flux for dark matter search experiment is important since seasonal modulations
of environmental neutrons that interact with the nuclei in the NaI(Tl) target/detector crystal array might mimic the WIMP-induced signals. To understand environmental neutron flux for the COSINE-100 experiment systematically, we develop a Neutron Monitoring Detector using liquid scintillator and study its energy response, characterize its pulse shape discrimination properties, and measure background from the detector. Here, construction and tests of the Neutron Monitoring Detector for the COSINE-100 experiment are presented.