Simulating grain and straw nitrogen concentration is of paramount importance in cropping systems simulation models. A simple model to partition nitrogen between grain and straw at harvest for barley (Hordeum vulgare L.), wheat (Triticum aestivum L.), maize (Zea mays L), and sorghum (Sorghum bicolor Moench) is presented. The principle of the model is to partition the aboveground nitrogen at physiological maturity based on the relative availability of biomass and nitrogen to the grain. The inputs for the model are the harvest index, representing the relative availability of biomass to the grain, and the aboveground nitrogen concentration at harvest, representing the availability of nitrogen. The model has five parameters, of which four (the maximum and minimum achievable grain and straw nitrogen concentrations) are readily available, and only one, the empiric parameter C, requires calibration. The model was calibrated and tested for these four species without differentiating genotypes within species. The testing included diverse experiments in wheat; comparing observed and estimated grain nitrogen concentration, the relative root mean square error (RMSE) ranged from 3 to 10% (five experiments) and was 31% in one experiment in which the estimations consistently exceeded the observed values. For barley, maize, and sorghum the data availability for testing was limited but still the model performed well with relative RMSE of 7, 7, and 18%, respectively. Therefore, the model proposed seems to be robust. It remains to be determined if the parameters and the method are useful to discriminate genotypic differences in grain nitrogen concentration within a species, and if the method can be applied to legumes.