Dislocation is one kind of the most important structural defects in graphene sheets. In the present work, graphene dislocation is studied through the computational model of a dislocation dipole with two opposite cores. Strain fields around the dislocation cores are calculated at atomic level according to virial strain concept, which is derived from the coordinates of discrete atoms and the symmetry of crystal lattice. Results show that both infinitesimal and finite virial strains are consistent with the predictions from elastic theory very well, thus the accuracy of virial strain analyses is validated. However, there is a slight difference between the two strain fields. The core area of the dislocation dipole can be outlined by the strain differences, which can be influenced by the neighboring atomic structure. Compared with the second nearest neighboring environment, the nearest neighboring environment has a smaller volume, therefore can be influenced by the local deformation state more easily. Finally, a new method to visualize the cores is proposed based on the displacements of graphene atoms. The location and structure of the dislocation dipole predicted by this new method are in agreement with the strain analyses excellently, and the structure changes generated by ripples can also be described.