Abstract: The change of glutamine synthetase (GS) isoform was determined by Native-PAGE and the activity staining as well as Western blotting. The activity changes of GS, NADH-dependent glutamate synthase (NADH-GOGAT), and NADH-dependent glutamate dehydrogenase (NADH-GDH) were also analyzed by enzyme assay. The results indicateded that only one form of GS isozyme was detected during the germination of cushaw seed and the different development stages of the cotyledon grown in light or in darkness. GS activity was increaseed with the germination of the seed untill the cotyledon to begin green and then was reduced gradually. The tendency of the activity change of NADH-GOGAT was similar to GS. The activity change of NADH-GDH was contrary to that of GSand NADH-GOGAT. The results suggested that the expression of GSisoform was not effected by the treatments of light and darkness in the experiment conditions. GS/GOGAT cycle may play a main role for assimilating ammonium during the germination of the seeds and the initial stage of the cotyledon development, and the cycle was weaken and GDH pathway was strengthened after the green of the cotyledon by comparing these enzyme activities.