An increase in the production per area of sunn hemp (Crotalaria juncea L.) biomass in order to expand its beneficial effects as green manure is an objective for the agronomic management of this species. Three experiments were performed during consecutive years to test the following plant densities per m-2: 10, 20, 30, and 40; 25, 35, 50, and 100; 25, 50, 75, and 100. For each density the spacing between the sowing rows and withinthe rows was equal. The experiments were conducted at Mossoro, RN, Brazil, with a randomized complete block design of four replicates. For each experiment, we determined the shoot dry mass and root dry mass per hectare and the ratio between the values per hectare of shoot dry mass and seed rate. Data were subjected to analysis of variance (F test) and regression analysis. The increasing linear behavior of shoot and root biomass of sunn hemp as a function of plant density establishes a recommendation of 100 plants m-2 with equal spacing in the sowing row and between rows. However, the amount of dry mass produced by each kg of seeds decreased to the density of 90 plants m-2. The amount of biomass produced was limited by the length of the vegetative phase of sunn hemp during day length conditions at Mossoro.