To measure the two/three-dimensional velocity vectors in a liquid metal flow, a new neutron imaging method was applied. By using a conventional transmission neutron imaging (neutron radiography) only two-dimensional information of tracer particles can be obtained. In this study, three-dimensional position was measured by analyzing the image unsharpness (blurr) of tracer particles depending on the distance between the particle and the imaging screen in the neutron beam direction. Neutron imaging has been performed at the Kyoto University Research Reactor and the variation of image unsharpness of particles was investigated.