Abstract: This paper studies the characteristics of inertial gravity wave critical layers in the mesosphere with mobile SOUSY VHF radar observations at Andφya (69° N,10°E)Norway June 1987.Data analysing shows, near critical layers, inertial gravity waves suddenly decay,wave energy are absorbed by background wind, the horizontal and vertical propagation directions may change repidly through critical layers,that indicates a downward prapogating energy source will be there at critical layer.The observations further show there is a maximum echo intensity at critical layers, strongly suggesting that critical layers play an important role on the generation of radar echo.