The spatial movements of yellowfin tunas around the fish aggregating devices which were anchored below 40 meters water with a VEMCO VR2 receiver were investigated using ultrasonic telemetry tags V9P (VEMCO VP9-2H-S256). The experiment was conducted in the Shiao-Liu-Chiu Island of Taiwan from July to September, 2007. A pair of VEMCO VR2 receivers was separated by 400 meter distances in west of south sea ,Taiwan. A total of 9 tagged fishes with different sizes were monitored for maximum 19 days. Preliminary results showed that juvenile fish stayed longer and closer within FADs than the adult fish. Fish could move vertically to as low as 160 meters at daytime. They avoid the surface water where the temperature reach 30oC. In nighttime, juvenile fish that has fork length smaller than 55cm generally moves to the surface mixed layer. On the horizontal movement, there are three types. A) fish stays at FADs site. B) Fish move in and out FADs sites for a few hours period. C) fish moves away from FADs site for a long times (maybe 2~3 days). Sometimes juvenile fish would have B) or C) moved away from the FAD devices during dawn and dusk, and back at daytime. But this would not occurred with large fish. We estimated the attraction radius of FADs at least 1 km.