Small angle neutron scattering (SANS) was applied to determine the nano-size defects of thin chrome layers to find the defect size and number of distribution without breaking the thin chrome layers. Most of defect size of the trivalent chromium prepared in this test conditions is about 40 nm. The number of nano-size defects less than about 40 nm of the trivalent chromium layer increases with plating voltage at constant current density. From this study, SANS is proved as one of useful techniques to evaluate nano-size defects of thin film layer.