Abstract: 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.