Abstarct: A field experiment was conducted at Research Farm of Shahrood University of Technology, Shahrood, Bastam in 2010 to quantify some responses of cowpea to population density. Factorial arrangement of 5 densities (10, 13, 16, 19 and 22 plants m-2), and 2 cultivars (Mashhad and Dezful) were the treatments. The results indicated that end of linear growth stage happened earlier in low-density populations than in high-density ones. It was found a linearly decreasing trend in time to canopy closure with increasing population density. Generally, the time to canopy closure tended to be shorter for cultivar Mashhad as compared to cultivar Dezful. This superiority might decline the loss of water through evaporation and dominate the plant to weeds. Among equations, the segmented function could predict well the allocation of dry matter to stem and leaf. The changes in height of pod from the soil over growing season were again well predicted by segmented function. The pod height tended to show no change during early stages of development; then it decreased with time increment. It was found the quadratic relation between remobilization of stored pre-anthesis assimilates to grain and population density; at low densities, the remobilization was increased with increasing population density. The segmented and quadratic functions could describe well the relation of grain yield with population density for Mashhad and Dezful cultivars, respectively. The optimum population density was 10 and 15 plants m-2 for these cultivars, respectively.