The effects of nitrogen fertiliser on herbicide dose–response of weeds were investigated by measuring weed biomass after growth at a range of nitrogen levels and treatment with a range of herbicide doses. Increasing weed biomass at no-herbicide treatment (W0) and the response rate of the dose–response curve (B), with increasing nitrogen were successfully described by the linear model and the exponential model respectively. Conversely, decreasing ED50 value with increasing nitrogen was well described by the logistic model. A combined model was then developed by incorporating these models into the standard dose–response model to describe the interactive effects of herbicide dose and nitrogen levels on biomass of Brassica napus, Matricaria perforata, Papaver rhoeas and Galium aparine. The model developed allowed the systematic description of increased herbicide performance with increasing nitrogen. The model was also used to predict weed biomass as affected by both herbicide doses and nitrogen levels. The mathematical relationships between herbicide dose–response and nitrogen levels may also be applied to the crop–weed competition model and then to decision making for optimum uses of nitrogen fertiliser and herbicide.