Longitudinal　modes　of　ultrasonic　guided　waves　in　liquid-filled　pipes　with　viscoelastic　coatings　are　analyzed　theoretically　and　experimentally.　Dispersion　curves　of　longitudinal　modes　are　numerically　calculated.　Then　longitudinal　modes　which　are　suitable　for　defect　detection　in　liquid-filled　pipes　with　viscoelastic　coatings　and　their　frequency　ranges　are　given.　The　results　show　that　L(0,　1)　mode　at　the　frequency　range　of　0-50　kHz　and　unperturbed　L(0,　2)　mode　branching　with　low　attenuation　and　weak　dispersion,　such　as　L(0,　4)　mode　at　the　frequency　range　of　170-210　kHz,　are　suitable　to　detect　defects　in　a　25　mm-outer-diameter,　1.2　mm-wall-thickness,　water-filled　steel　pipe　coated　with　0.35　mm-　wall-thickness　epoxy,　whereas　longitudinal　modes　whose　energy　mainly　propagates　in　water　and　viscoelastic　coating　of　epoxy,　and　whose　dispersion　and　attenuation　are　significant　are　unsuitable　to　detect　defects　in　these　kinds　of　pipes.