Abstract: The basic principle of the laser longitudinal mode splitting is introduced briefly, and the Nd∶YAG laser longitudinal mode splitting produced by placing one and two quarter-wave plates in the cavity of a Nd∶YAG laser is investigated experimentally. The results show that each longitudinal mode is split into two orthogonally and linearly polarized modes, and the splitting magnitude (i.e., frequency difference or wavelength difference) is just equal to one half of the laser longitudinal mode interval if the end surface of the quarter-wave plate is perpendicular to the laser beam when a quarter-wave plate is placed in the cavity of Nd∶YAG laser at 1064nm. The mode splitting phenomenon can also take place when the Nd∶YAG laser cavity contains two pieces of parallel placed quarter-wave plates along the direction normal to the laser beam, and its mode splitting magnitude is dependent on the angle between the fast (or slow) axes of both quarter-wave plates. The magnitude can be linearly tuned in one longitudinal mode interval when the angle is adjusted between 0 to 90 degrees. The results obtained experimentally agree with those of the theoretical analysis quite well.