Cracking behavior under multiaxial fatigue has been observed to be affected by not only stress multiaxiality but also metallurgical microstructure. In copper which was used in this study, cracks were observed to be nucleated and to grow along grain boundaries. It was also found that the crack growth was assisted by coalescence among initiated cracks. These observations suggest that the direction of the main crack might be determined by the distribution of grain-boundary cracks. A modeling of grain-boundary cracking is required to establish the way to predict the direction of the main crack, depending on stress multiaxiality. In this analysis, two components of shear deformations on the grain-boundary plane were taken into account to describe incompatibility along the grain boundary. New parameters based on two shear deformations were proposed to represent the orientational distribution of grain-boundary crack. Calculated results were composed with experimental crack distribution during biaxial fatigue tests with constant and variable strain-biaxialities.