During manufacturing processes of thin materials (called webs) such as papers, photographic films, and magnetic films, the free edges of a web can vibrate violently due to the airflow in the cross direction. A traveling-wave analysis was done based on linear potential flow theory, and the critical flow velocity, wave velocity, wavelength and flutter frequency were predicted. Experimental verification was done in a wind tunnel built around a web-translating machine. Two laser-Doppler vibrometers were used for non-contact measurement of wave characteristics. For flexible webs, the theory and test results show good agreement. Test results for stiff webs, however, deviate from the prediction.