A frame averaging circuit adapted for a color blood flow frame in a medical ultrasound imaging system. The frame averaging circuit comprises two sections, i.e., a recursive calculation section and a non-recursive section. The recursive calculation section employs an IIR filter in combination with a look-up table, and is adapted for dealing with the cases where no change occurs to the blood flow velocity direction. The non-recursive section compares the blood flow velocity, the energy and the energy prior to wall filtering of the color blood flow echo signal to various threshold values. Different from the recursive calculation section, the non-recursive section is employed when a change occurs to the blood flow velocity direction.
