Abstract: The affecting factors for determination of trace hydrogen in U Nb alloy are studied. The status of samples affects greatly the measured hydrogen content,which has the higher value for scrap sample than cylinder sample.The hydrogen content in cylinder is closer to real value in U Nb alloy.The hydrogen in U Nb alloy is mainly from surface adsorption.The results of hydrogen determined by vacuum thermal evolved method,in which the relative standard uncertainty is less than 28%,are in agrement with those by inert gas fusion method.