50 kHz and 200 kHz ultrasound were selected to study the transient and steady aggregation behaviors of hydrophobic particles in ultrasonic field, respectively. Different acoustic aggregation phenomena were observed by high-speed cameras, and the differences between the transient and steady aggregation in position, velocity, form, quantity and intensity were compared. The formation mechanisms of transient and steady aggregation are analyzed from the perspective of acoustic radiation force and cavitation. The results show that 50 kHz and 200 kHz acoustic field cause transient and steady acoustic aggregation, respectively. There are great differences in the behavior and formation mechanism of the two acoustic aggregation modes.