Ultrasonic treatment is an effective but less practiced dynamic nucleation method for grain refinement of molten or solidifying alloys. In this study the influence of ultrasonic treatment on the microstructural features and mechanical properties of AZ91 magnesium alloy was investigated. Ultrasonic waves were directed to the surface of the molten or solidifying alloy using an ultrasonic device with frequency of 20kHz and nominal power of 600W. The parameters investigated included the intensity and duration of ultrasonic treatment as well as the casting temperature in the mushy zone of the alloy (solid fraction before pouring). For investigation of the effect of ultrasonic intensity, the melt was ultrasonic treated for 5min at 700 o C at 0, 20, 40 and 60 percent of the nominal power of the device and was then poured in a sodium-silicate sand mould. For study of the effect of time of ultrasonic treatment, the melt was ultrasonic treated at 30% of the nominal power of the device for 0, 2 and 15min at 700 o C and was then poured in a sodium-silicate sand mould. For investigation of the effect of solid fraction before pouring, the alloy was melted and then continuously cooled inside the furnace. The ultrasonic treatment with 40% of the nominal power of the device was started from 605 o C (i.e. about 7 o C super heat) and continued until reaching various temperatures in the mushy zone of the alloy and then the samples were quenched in water. Similar experiments were carried out with out ultrasonic treatment. Cast samples were then cut and their microstructural features and mechanical properties were investigated. Size and morphology of ?-Mg dendrites and intermetallic phases such as Mg 2 Si, MnFeAl(Si) and Mg 17 Al 12 were characterized using image analysis techniques. Experimental results indicated that with increasing the ultrasonic power or ultrasonic treatment time, the size of grains and phases decreased while their sphericity increased. Tensile strength and elongation of the samples increased simultaneously with increasing the ultrasonic treatment power or time. Results of the continuous cooling experiments indicated that at each casting temperature (solid fraction) the size of grains and intermetallic phases decreased and their sphericity increased by ultrasonic treatment compared to the conventionally cooled samples. Also with increasing the casting temperature within the mushy zone under ultrasonic treatment, the average length and width of Mg 17 Al 12 either in the quenched zone or between the primary dendrites decreased. At each casting temperature (solid fraction), Key words: Ultrasonic treatment, AZ91 magnesium alloy, Microstructure, Mechanical properties