The size and number of nano-size defects of thin trivalent chrome layers were determined by small angle neutron scattering (SANS) without breaking the thin chrome layers. Most of defect size of the trivalent chromium prepared in this test conditions is in the range of about 40nm. The number of nano-size defects less than about 40nm 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.