A model is developed to predict the optimal shoot weight ratio (SWR: the fraction of shoot weight to total plant weight) for maximizing the relative growth rate (RGR) at a given specific root activity (SRA: the rate of nitrogen accumulation per unit root weight) for a plant undergoing steady state exponential growth. The model is based on two assumptions: (1) specific shoot activity (SSA: the rate of dry matter accumulation per unit shoot weight) is a function of shoot nitrogen concentration and (2) root- and shoot-nitrogen concentrations are linearly related to each other. The model predicts that at high nitrogen availability, the optimal SWR increases with increasing SRA to maintain a shoot nitrogen concentration at which the SSA is just saturated at a maximal level. At low nitrogen availability, where SRA is less than a critical value, the model predicts a constant SWR as an optimum irrespective of SRA, which causes a reduction of shoot nitrogen concentrations with decreasing SRA. Two grass species, Holcus lanatus L. and Festuca ovina L., were grown in a flow-culture system. At medium and high N, both species showed higher SWRs than expected optima. At low N, there is a time lag for H. lanatus to reduce the SWR from an initial high level following germination to an optimal level. In contrast, slow-growing F. ovina achieves optimal SWR soon after germination and maintains it during growth, irrespective of temperatures.