The KIMS collaboration is an experimental group searching for the Weakly Interacting Massive Particle (WIMP) which is one of the strongest candidates of dark matter. In a WIMP search experiment, the neutron is an important background because the nuclear recoil signal of a neutron is indistinguishable from that of a WIMP. We have constructed a 20-liter liquid scintillation detector to measure the neutron background. We present the performance of the neutron detector, as well as its structure and electronics system. We have achieved a good neutron/gamma separation and a good position measurement. A position dependent energy calibration has also been performed to correct the nonlinear response of the detector for different particle incident positions.