Abstract: The dispersion character of medium-scale Travelling Ionospheric Disturbances (TIDs) observed at Millstone Hill (288.5°E, 42.6°N) is statistically studied. The result shows that, on frequency-velocity plane, there exist several peaks of TIDs occurrences. Further analysis shows that the observed dispersion characteristics matches the theoretical dispersion relations of guided gravity waves. That is to say some high frequency TIDs, may be interpreted as the fully guided waves between the ground and the bottom of thermsphere, and the low frequency TIDs may be related to guided waves L₁, W₁ and W₄.