Soybean [Glycine max (L.) Merr.] is an economical important crop because its  protein and oil is suitable for animal and human consumption. The development of varieties with improved yield, protein and oil content and other agronomic characteristics is highly desirable. In conventional breeding, environmental factors have an influence on selection efficacy and they become the primary obstacle in genetic improvement. Plant density can be considered as an environmental factor that can induct variance and affect genotypic expression. In 2010, thirty-one soybean genotypes were evaluated under ultra-low plant density (1.4plants/m²) according to the honeycomb arrangement and high-plant density (33 plants/m²) in randomized complete blocks at two different soil-type environments in Carbondale IL. Comparison of two different plant densities and selection of the one that introduces the lowest environmental variance was based on the calculation of coefficient of variation for seed yield. In the course of time, determination of the optimal plant density for evaluating genotypes may result to a more effective selection and development of varieties that exhibit both high and stable productivity.