The predominant strain producing mechanism in superplastic materials is attributed to grain boundary sliding (GBS). Though there have been many attempts to model GBS, most of them are heuristic and not based on the microscopic deformation at the scale of grain boundaries. Lack of such models has hindered the progress of a methodical development of alloying processes. Hence an attempt is made to develop a constitutive model of grain boundary using atomic simulation results. Detailed atomic simulations are carried out on aluminum bicrystals of Sigma3 and Sigma9 coincident site lattice boundaries to obtain characteristic behavior of grain boundary sliding/migration. Using the results thus obtained a constitutive model based on cohesive zone approach is developed to predict the behavior of grain boundaries under the external loads and hence GBS. This constitutive model is implemented in a finite element method (FEM) code to analyze the GBS/migration at a continuum scale.