This study was carried out to investigate the effects of four diazotrophic bacteria:  Enterobacter asburiae, Stenotrophomonas maltophilia, Bacillus safensis, and Rhizobium freirei, which were isolated from rubber roots planted in Vietnam, on rubber trees at the nursery stage. The results showed that these bacteria had significant influences on the germination rate (P ≤ 0.01); the germination rate was affected highest by Bacillus safensis and lowest by Rhizobium freirei. In contrast, the four bacteria had no significant effect on the germination percentage (P > 0.05). The best colony forming unit was 109 CFU/pot. The bacteria also created statistically significant differences in root length, stem length, height and dry weight (P < 0.01). By using 15N dilution technique, Enterobacter asburiae and Stenotrophomonas maltophilia demonstrated a biological nitrogen fixing contribution to the rubber trees with the ratio of nitrogen drived from air (%Ndfa) of 46.43-47.62% higher than Bacillus safensis and Rhizobium freirei with the %Ndfa of 35.71-38.69%.