Salinity and drought stress can significantly effect plant yield in arid and semi-arid regions. Legume-rhizobium symbiotic relationships can also be influenced by those limiting factors. It is well known that the host plant inoculation by native strains with high efficiency has a positive effect on plant yield and on biological nitrogen fixation process. The main aim of this investigation was to evaluate the salinity and drought tolerance of 82 isolates of Sinorhizobium meliloti isolates which collected from Kerman province in Iran. Salinity and drought tolerance of all isolates were examined in liquid TY media containing 0, 100, 200, 300 400, 500, 600 and 650 mM. NaCl and 0, 203, 298, 373, 438, 496, 548 and 573 g/L of Poly Ethylene Glycol (PEG6000) respectively. This experiment was carried out by using a factorial model in completely randomized design. The results showed that salinity and drought tolerance among isolates was significantly different. All isolated were grouped in three clusters: sensitive, semi-sensitive and tolerant based on their growth rate in TY media containing different concentrations of NaCl and PEG6000. The results also showed that all tolerant isolates excreted more exopolysacharides compared to the sensitive and semi-sensitive isolates. Based on salinity and drought experiments, four isolates of Sinorhizobium meliloti (S27K and S36K tolerant isolates, S109K semi-sensitive isolate, S56K sensitive isolate) were selected for plant inoculation which was under drought stress in green house condition. This experiment was carried out by using a factorial model in completely randomized design. The results showed that the yield of alfalfa increased 2.7 times higher than control treatment when alfalfa plants were inoculated by S27K and S36K isolates and yield of alfalfa increased 1.5 times higher than control treatment when alfalfa plants were inoculated by sensitive isolates S56K. There were not any significant differences between S27K and S36K isolates and positive control (no nitrogen limitation). Results showed that inoculation of alfalfa plants with high salinity and drought tolerant of Sinorhizobium meliloti bacteria can improve biological nitrogen fixation process and plant production under salinity and drought conditions (Figure 1). Key words: Sinorhizobium meliloti, alfalfa, Salinity, drought, exopolysacharid