Four interrelated field and greenhouse experiments were conducted to compare the intra- and inter- specific competition of Medicago sativa L. and Bromus tomentellus Boiss. in relation to soil properties. The effects of plant density and water stress on yield of M. sativa and B. tomentellus were studied in a greenhouse experiment with 18 treatments, three crop compositions ( M. sativa , B. tomentellus and a mixture of two crops), two plant density levels, three drought stresses and four replications. This experiment was conducted completely randomized block design. Results indicated that total yield of M . sativa monoculture was higher than mixed culture and higher than B. tomentellus monoculture. land equivalent ratio (LER) values were less than 1 for all mixed culture treatments, indicating the inter-specific competition between the two species. The biomass production per plant decreased with increasing density showing competition effects of resource utilization and water stress. Analysis of foliage and root biomass showed that M. sativa appeared to be more constrained by intra-specific than by inter-specific competition. Conversely, growth of B. tomentellus was more suppressed by inter-specific competition exerted by M. sativa than by intra-specific competition. Relative competition intensity (RCI) values were positive for B. tomentellus and negative for M.sativa , implying that competitive ability of M.sa tiva was higher than B. tomentellus in the mixed culture. A field experiment was also conducted to compare stand establishment, forage production and quality of M. sativa and B. tomentellus species using factorial arrangement of treatments based on completely randomized design with five replicates in Natural Resources Research Station of Fereydoon-Shahr, Isfahan province. Treatments consisted of three crop compositions ( M. sativa , B. tomentellus and a mixture of two crops) in 2.1×3.5 m 2 plots area, three density levels of low (4 rows, row spacing 70 cm), medium (8 rows, row spacing 30 cm) and high (16 rows, row spacing 14 cm) and two planting dates (spring and autumn). Results demonstrated that autumn and spring planting of B. tomentellus and spring planting of M. sativa were successful. The forage production of M. sativa and B. tomentellus in intercropping was significantly (p 0.05) higher than in monoculture. In intercropping treatments, LER values, indicated that land-use efficiency in intercropping was higher than in monoculture. The forage quality in intercropping was significantly higher than in monoculture. According to the results, intercropping of M. sativa and B. tomentellus with 8 rows density (30 cm row spacing) is recommended in poor dryland fields of Fereydoon-Shahr and similar areas. Plant available water (PAW) and integral energy ( E I ) values for M. sativa and B. tomentellus were determined and calculated in five soils collected from semi-steppe rangelands of Fereydoon-Shahr. The experiment was arranged in a factorial design within main completely randomized design with five replicates. The PAW values for M. sativa were greater than those for B. tomentellus in all of the studied soils. The E I values for M. sativa were also greater than those for B. tomentellus in some of the studied soils. It was concluded that M. sativa was able to tolerate higher soil matric suctions. M. sativa with a well-developed taproot system could apply greater energy to take up a unit mass of PAW than B. tomentellus . Results showed that calculating PAW and E I only from soil properties is not error-free. The PAW and E I are dependent on soil properties and plant species in rangeland plants. The E I quantifies the energy required by a plant to remove a unit mass of soil water over the PAW range and can be used as an indicator in intra- and inter-specific competitions in rangeland plants. Keywords: Intra- and inter- specific competition, Mono- and mixed culture, Medicago sativa L., Bromus tomentellus Boiss, Plant available water, Integral energy.