This paper studied the effect of ultrasonic waves on sedimentation characteristic of activated sludge and there causes on filamentous growth, as well as its effects on the sludge ability dewaterability. The power of ultrasonic waves was set at (0.35W/cm2) while the frequency and contacts time was varied to (31،44،55 KHz) (10،20،30،40 min) respectively. An instrument with (0.5 L) in volume was used to applied ultrasonic waves, it was connected by plastic tube with a completely mixed activated sludge reactor. Testes shows that the relationship between contacts time of ultrasonic and sludge volume index (SVI) was a sagged curve with an optimum point, that’s means there was a specific contacts time to achieved an optimum sedimentation characteristics at specific frequency. As well as Testes shows that the relationship between contacts time and specific resistance (r) was a linear in first order, that’s indicates the ability of dewaterability of sludge decreased with the increasing of ultrasonic contacts time. Also results shows that the effect of ultrasonic waves on activated sludge related conversely with its frequency.