Light hydrocarbon (methane, ethane, propane, butane and CO 2) test and C isotopic analysis of CO 2 are conducted for over 100 lower-layer atmospheric samples from the East China Sea slope and the Okinawa Trough. The results show that the lower-layer atmosphere mainly consists of CO 2 and then of CH 4, and the CO 2 concentrations are calculated to have a high average value of 0.87 ω/10 -2,about three times that of the regional background (0.3 ω/10 -2). The result also shows that the average value of C isotope -20.8×10 -3 is given to the CO 2, inferring that it is inorganic gas. Thus,for the future’s work in the Okinawa Trough, special attention should be paid to CO 2 hydrate, which is very possibly an important hydrate type. Light hydrocarbon (methane, ethane, propane, butane and CO 2) test and C isotopic analysis of CO 2 are conducted for over 100 lower-layer atmospheric samples from the East China Sea slope and the Okinawa Trough. The results show that the lower- layer atmosphere mainly consists of CO 2 and then of CH 4, and the CO 2 concentrations are calculated to have a high average value of 0.87 ω / 10 -2, about three times that of the regional background (0.3 ω / 10 -2) . The result also shows that the average value of C isotope -20. 8 × 10 -3 is given to the CO 2, thus inferring that it is inorganic gas. Thus, for the future’s work in the Okinawa Trough, special attention should be paid to CO 2 hydrate, which is very possibly an important hydrate type.