This study investigated the characteristics of sediment immersion activity in order to utilize them as a basis for suitable site selection when releasing cockle, Fulvia mutica spat. The cockle, F. mutica spat were spawned from natural broodstock in April, and the cockle spat (shell length 17.96 ± 4.78 mm) were reared in a 2 ton FRP square tank (300 × 100 × 70 cm) for 185 days in a flow-through system. To investigate the sediment immersion activity characteristics, 30 were placed in each sediment grain size of fine sand (0.12 ± 0.03 mm), medium sand (0.93 ± 0.24 mm) and coarse sand (2.69 ± 0.67 mm), and the immersion rate and immersion time of each individual spat were investigated. As a result, the in rate of each 10-20 mm cockle spat was more than 90% within 60 minutes except for the coarse sand, and the immersion time was more than twice as long.