Technical Abstract: Simulating grain (Ng) and straw (Ns) nitrogen concentration is important in cropping systems simulation models. In this paper we present 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). 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 (HI), representing the relative availability of biomass to the grain, and the aboveground nitrogen concentration (Nt) 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 parameter C, requires calibration. The model was calibrated and tested for these four species without differentiating genotypes within species. In wheat, comparisons of observed and estimated Ng had relative root mean square errors (RMSE) that ranged from 3 to 10% (five experiments) and was 31% in one experiment in which the estimated Ng 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. We concluded that the proposed model seems to be robust. It remains to be determined if the parameters and the method are useful to discriminate genotypic differences in Ng within a species, and if the method can be applied to legume crops.