Concentric-ring pattern is observed in an Ar/air mixture dielectric barrier discharge. The discharge images within one half voltage circle are taken by an intensified-charge coupled device camera, indicating that the discharge filaments are the basic units of the concentric-ring pattern. By comparing the six instantaneous images corresponding to three successive positive and negative half voltages, it is proved that the concentric-ring pattern seen with naked eyes is formed by the numerous discharge filaments located at different positions during successive acquisition intervals. With applied voltage increasing, concentric-ring pattern can transform into spiral, and then into concentric-ring pattern again. By analyzing the features of formation and transformation of these two patterns, it is inferred that the two patterns have similar dynamic mechanisms. Discharge powers of concentric-ring pattern and spiral are calculated respectively, and the results show that the power increases linearly approximately with applied voltage increasing. The correlation coefficients of concentric-ring pattern are compared with those of spiral, and the results show that the correlation coefficient of concentric-ring pattern is relatively low and irregular, while the correlation coefficient of spiral is relatively high and has an oscillatory characteristic.