The present invention relates to an ultrasound system and a method of forming a Doppler mode image. The ultrasound system comprises a transmission/reception unit operable to transmit/receive ultrasound signals at a pulse repetition frequency (PRF) to form an n number of Doppler signals. The ultrasound system further comprises a signal processing unit. The signal processing unit computes a first mean frequency based on an nth Doppler signal and an (n−1)th Doppler signal and a second mean frequency based on an nth Doppler signal and an (n−k)th Doppler signal, wherein k is a positive integer equal to or greater than 2. The signal processing unit estimates a Doppler frequency and compensate for aliasing of the Doppler signal based on the first and second mean frequencies to produce Doppler signal with the aliasing compensated. An image processing unit operable to form a Doppler mode image based on the Doppler signals produced by the signal processing unit.