Abstract: The axial power flow distribution and attenuation coefficient distribution of ultrasonic longitudinal guided waves in viscous liquid-filled pipes were calculated and analyzed. The optimal location, optimal mode and its frequency-thickness products (fd) for the test pipes filled with viscous liquid were chosen according to the axial power flow distribution. The results showed that for NDT of viscous liquid-filled pipes, the optimal frequency-thickness products and testing locations of using each mode were L(0,1) mode under 0.07 MHz·mm for inner walls, and L(0,2) mode in 0.09~0.16 MHz·mm for everywhere, L(0,3) mode in 0.18~0.28 MHz·mm. Axial power flow distribution can be effective in choosing the optimal guided waves and its frequency-thickness products of NDT.