The aim of the study was to evaluate a weed infestation level of legume-cereals mixture at diverse percentage of pea seeds in organic system. The study was conducted in 2011-2012 in split-plot method in four replications on good wheat complex at the Institute of Soil Science and Plant Cultivation - Agricultural Experimental Station Grabów (Mazowieckie Voivodeship). Mixtures of two pea varieties (Wiato, Tarchalska) with spring wheat at diverse percentage of legume crop (40, 60, 80%) were compared. The study included yielding and the qualitative and quantitative analysis of weed infestation. The description of weed association structure by ecological indexes: Shannon’s diversity and Simpson’s dominance were done. The study showed that mixture with 40% of pea was the most competitive in relation to weeds. The lowest weight of weeds was noted in those mixtures. In both years of study, bigger weight of weeds was observed in the second term of estimation. The analysis using Shannon' s diversity index revealed the biggest weed flora in the first year of study in mixture of wheat with 60% of pea, Wiato variety and in the second year in mixture with 80% of pea, Wiato variety and 60% of Tarchalska variety. The smallest number of weeds in the first year of study was in mixture with 80% of pea, Wiato variety. In the second year, Simpson’s dominance index showed domination of one species in mixture with 40% of pea, Tarchalska variety.