The aim of this study was to evaluate yield of serradella sown in spring cereals harvested on different dates, grown in organic farm. The experiments were performed in the years 2011 and 2012, in Agricultural Advisory Centre Szepietowo (podlaskie province). It was conducted in the split-plot design, with the control objects, in four replications. In the scheme of the experiment two factors were included: species of cereals: oats, barley and the cover crop harvesting time: milk-dough stage, complete maturity (grain). Cereals without serradella were sown in a sole-crop sowing on the control objects. The yielding of serradella, cultivated as undersown crop in oats or barley, was differentiated by cereal species and its harvesting time. The higher yield was obtained from serradella sowing in cereals harvested at milky-dough stage than complete maturity. Regardless of the cover crop harvesting time, higher yield of serradella in barley than oats was obtained. In the year with smaller amounts of rainfall (2011), the average dry matter yield of barley and oats was similar but in wet year (2012) oats yielded significantly better. The yielding of oats in pure sowing was much greater than barley, but with undersown serradella was similar. In organic farming higher level of grain yield was obtained from oats than barley, regardless of the method of sowing. The yield of oats in sole-crop sowing and with undersown serradella was similar, but yield of barley with undersown serradella was significantly lower than in pure sowing.