Abstract: Describes an investigation of the ultrasonic longitudinal guided waves propagation characteristics in viscous liquid-filled elastic pipes is gives. The dispersion characteristics were computed on the assumption that the frequency is real number and wavenumber is complex number. Then the displacement amplitude distributions and the attenuation distributions of guided waves were obtained and analyzed. Finally, the optimal location and frequency-thickness products of testing for pipes are chose by using them. The results shown that below 0.07 MHz-mm, using L(0,1) mode to detect the defect of the pipes filled with viscous liquid should be more effective, especially for the inner wall of pipes. The optimal detect ranges of frequency-thickness products of L(0, 2) and L(0,3) modes are 0.09-0.16 MHz-mm, 0.18-0.28 MHz-mm, respectively.