Nitrogenase is the enzyme for nitrogen fixation (nif), encoded by nijH gene. Based on nifH gene sequence of Pseudomonas stutzeri A1501 in the data bank (CP000304.1), PST3422 primers were designed to identify niflH genes of P. stutzeri isolated from rhizospherc of rice in the Mekong Delta. 15/15 P. stutzeri isolates had the ability to synthesize amonium in Burk N-free and 4 isolates were selected for further confirming the present of nifH gene by PCR technique. Partial nifH gene sequences (580 bps at 5' end) of 4 isolates (PS1, PS2, PS3, PS4) and reference strain (PS5: P. stutzeri
A1501) were applied for assessment of homologous level in nucleotide by BLAST N, BioEdit and building phylogenetic tree by MEGA 5.0. The result showed that all 4 partial genes from isolates had the indentity rate 98-99 percent with nifH of P. stutzeri A1501 strain and PS2 and PS4 isolates had closer relationship with reference strain A1501 while PS 1 and PS3 isolates were arranged in another branch. Both PS1 and PS4 isolates were tested nitrogen fixation ability on high-yielding rice in pots experiment. The result showed that grain yield of inoculated rice plus 50 percent N treatment did not differ w:ith grain yield of positive control treatment (100 percent N, without inoculation).