Abstract: For the inorganic-origin natural gas, especially the carbon dioxide gas, its accumulation in the deep-seated part of basin and inside mountain chain is related to cap rocks, which the tectonic decollement rifted rock (mylonite) is an ideal rock type．The ultrasonic method is optimum for detecting the, sealing effect of cap rock because it is effective, economic and convenient. The ultrasonic wave refers to the elastic wave propagating in continunm with its frequency over the hearing threshold value of human beings．The method is tenable theoretically, and is useful in study the anisotropy of rock’s wave velocity by means of ultrasonic wave, analyzing the wave velocity anisotropic body that shows oil and gas-bearing bed and sealing cover, and judging the migration direction of oil and gas. However, the study of wave velocity related to sea ling cover based on mylonite has not been reported so far. In this paper, the wave velocity of mylonite is detected and calculated. The result indicates that the wave velocity of mylonite is closely related to rock fabrics, tectonic stress field and accumulation-forming conditions. The wave velocity is negatively related to porosity(Vpz=5.96～ 0.98φ) and postively to sealing performance. The rock shows evident anisotropy, the minimum wave velocity direction indicating the sealing trend while the maxmum the favourable direction for migration of oil and gas.