The square pattern is observed in dielectric barrier discharge in argon for the first time. The hexagon pattern and the mixture of hexagon and square structure are also observed. The square pattern can remain stable for several tens of minutes. The spatio-temporal dynamics of square pattern formation is investigated by optical method. The measurement results show that the square pattern is an interleaving of two square patterns, which is the reason of the stability of square pattern. (C) 2003 Elsevier Science B.V. All rights reserved.