The transverse modes of a high power transverse-flow CO2 laser are on-line measured by using a laser beam analyzer. The temporal evolution of the transverse modes is observed also. The output beams are characterized with “double mode”, i.e., two higher-order modes spacing 8 mm between the mode centers, and overlapping unsymmetrical configurations. There is evidence to whow that such “double mode” arises from a misaligned axes of the resonator consisting of two tandem resonators, and a distorted cathode-copper tube. The transverse mode are more degenerated into complicated shapes as the lasing operating duration increases. An initial investigation shows that the temporal evolution of the modes depends on the gaseous component varying in the resonator during the lasing operation.