A computer simulation model (PASTMODII) of the yield response of grass/legume mixture pastures to soil water, temperature and nitrogen was created and tested. PASTMODII was built upon the framework of a single variety grass model PASTMOD, to include root growth, soil temperature and nitrogen effects. PASTMODII operates on a daily time step over a growing season and compiles harvest and season total yields. It consists of five sub-models: root mass growth and distribution, soil water content, soil temperature and heat flow, soil nitrogen dynamics, and grass and legume growth. Most of the model components were adapted from algorithms validated by previous research. However, equations for modeling the following were introduced: attributing transpiration to each separate species, calculating nitrogen fixation and uptake by legumes and grasses separately, calculating total photosynthate production in a mixed canopy and partitioning this between grasses and legumes, dry matter removal from each species due to preferential grazing, and root growth and distribution in the soil. The modeled results were compared to a grazed research study on an orchardgrass-birdsfoot trefoil mixture at the Caine research dairy in Cache Valley of northern Utah, and with a mechanically harvested similar mixture at a high elevation site near Randolph, Utah. Although the model matched the measured results reasonably well lending credence to the methodologies used, additional work needs to be done to calibrate the model input parameters.