Periodic boundary conditions (PBC) are a set of boundary conditions that can be used to simulate a large system (i.e. bulk material) simply by modeling a finite Representive Volume Element (RVE). PBC has been favored among many researchers and practicing engineers in the study of various materials. Unfortunately it remains vague on how PBC should be applied properly in FEA packages such as Abaqus. In this article, we explicitly show the detailed procedures one could easily follow to define PBC in Abaqus through a simple example which is given in the forms of input file. A robust Matlab scr ipt which can be used to pair a large number of randomly distributed nodes on two opposite surfaces of a 3D RVE is also supplied to facilitate easy application of PBC.