Aim: Plant growth promoting rhizobacteria (PGPR) inluences plant health and productivity by various ways. A good selection of a PGPR strain to be needed for biofertilizer production especially red onion (Allium ascalonicum) cultivated on Arenosols. The objectives of this study were isolation, selection and evaluate good PGPR strains to red onion cultivaed on arenosols. Methods and Results: Nitrogen-fixing and phosphate-solubilizing bacterial diversity and population dynamics in the Arenosols rhizosphere of red onion (Allium ascalonicum) grown in Vinh Chau district, Soc Trang province, the Mekong Delta, Vietnam was studied. Arenosols rhizosphere samples were taken Physical and chemical characteristics of soil samples in four sites of this region and population of nitrogen-fixing and phosphate-solubilizing bacteria were determined by drop plate count method together with 16S rRNA gene fragments amplified from DNA using eubacterial universal primers (27F and 1492R). A total of 63 isolates were isolated on two media (Burk’s N-free and NBRIP) and all of them have ability of nitrogen fixation and phosphate solubilization together with IAA biosynthesis. Population of nitrogen-fixing and phosphate-solubilizing bacteria correlated with soil pH closely (PGamma-Proteobacteria. Conclusion: From these results showed that two strains (Enterobacter ludwigii N6b and Pseudomonas sp. N4b) revealed promising candidates with multiple beneficial characteristics and they have the potential for application as inoculants adapted to poor soils and red onion plants because they are not only famous strains but also are safety strains for agricultural sustainable.