The semi-solid TiAl3/A356 aluminum composite was prepared by ultrasonic vibration treatment(UVT). The effects of UVT temperature, time and power on the microstructure of semi-solid slurry of TiAl3/A356 composite were studied by using SEM and X-ray diffraction techniques. The results revealed that the size of primary α-Al particles increased with the decrease of UVT temperature and power. The size of primary α-Al particles decreased at first and then increased with the increase of UVT time. When treated for 60s at an UVT temperature of 608 ℃ and an UVT power of 1KW, a good primary α-Al particle morphology with an average size of 62 μm and a shape coefficient of 0.8 could be obtained. The mechanism involved in the development of microstructure is the result of increase of nucleation rate and undercooling caused by a cavitation and acoustic streaming. Furthermore, the in situ TiAl3 particles have strong ability to nucleate α-Al particles.