Glutamine synthetase （E 6.3.1.2） from thermoacidophilic archaebacterium Sulfolobus acidocaldarius was purified to 25 fold by DEAE-Sepharose and Sephacryl S-300 column. The N-terminal amino acids were determined as PGLPKNEHEALEFLKSNNIKWVDLQ, also one consensus sequence of a conserved region TFMPKP(I/L/F)(F/P/Y)(G/R) was found by using alignment of other glutamine synthetase sequences from archaebacteria, a pair of primers were determined according to the above two sequences. The PCR was processed by using S.acidocaldarius genomic DNA as template and a DNA fragment about 780bp PCR product was achieved. After cloning and sequencing of this fragment, the DNA sequence could be translated into a continuous protein sequence which showed high identity to the S.solfataricus GS sequence. After Southern blot of genomic DNA digested by different combination of restriction enzymes using above DNA fragment as probe, a 2.4 kb fragment digested by BamHⅠ/HindⅢ was cloned into pBluescript KS⁺ plasmid, after colony hybridization, the positive was chosen and a 1.5 kb complete glutamine synthetase gene was sequenced, the gene was then cloned into PET3C plasmide and was induced, the activity of GS was determined. The thermostability of this enzyme showed that it is indeed from thermophilic protein.