In the absence of herbicides, cover crops play an important role for weed management in organic than in conventional farming. Therefore, in order to evaluate the effects of cover crop management and plant density of buckwheat (Fagopyrum esculentum) as cover crop on weed suppression, a field experiment was conducted at the Research Farm of University of Zanjan, during the growing season of 2011. The experimental design was a randomized complete block design on the factorial basis with three replications per treatment. The experiment included cover crop management (dead mulch and living mulch) and cover crop plant density (0, 36, 78, 100 plants m^-2). A plot also assigned as weed free check (sunflower monocropping). It was observed that weed biomass significantly affected by buckwheat density (P≤0. 01). However, weed biomass did not affected by cover crop management. In other words, dead or living mulch had similar effects on weed biomass production. Buckwheat as living and dead mulch reduced weed biomass up to 91% and 88%, respectively, compared with no cover crop plots (the density of zero). There was also a negative correlation between cover crop density and weed biomass. Moreover, sunflower grain yield was also reduced with increasing buckwheat density from 36 to 100 plants m^-2. Buckwheat at the density of 36 plant m^-2 significantly decreased the weed biomass and saved grain yield, indicating that using this plant as cover crop can be more cost-effective and more environment-friendlier than chemical weed control in sunflower.