Compound-specific isotope analysis of N-15 enrichment of nitrogenous molecules by gas chromatography/isotope ratio mass spectrometry is a potential method for a quantitative access of nitrogen flow in plant nutrition and biosynthesis. Here, as a primary application of this method for plant nutrition studies, we demonstrate high sensitive detection of N-15 enrichment of amino acids in white radish cultivated either with very low level of N-15-labeled organic or inorganic nutrients, and evaluate the practical utility of this method.