The Al-5Zr master alloy was fabricated by direct melt reaction technique in the Al-K2ZrF6 system. Three-dimensional morphologies of the primary Al3Zr phase in Al-5Zr master alloy were acquired by the alkali corrosion and their formation mechanisms were analyzed. The effects of pouring temperature (1 250, 1 050, 950, 800 ℃) on the morphology, size and the amount of primary Al3Zr phase in the cooling process were investigated. The theoretical and experimental values of solid fractions were calculated by JMatPro software and area-method, respectively. The results show that the morphologies of Al3Zr phase in the master alloy display thick-plate, thin-flake, petal and cross-bridge shapes; and the formation of these morphologies are resulted from two-dimensional nucleus growth and constitutional supercooling. With the decrease of pouring temperature, the morphologies of Al3Zr phase are changed from thin-flake to thick-plate, the size and the amount for the thin-flake phase decrease, while the size and the amount for the thick-plate phase increase, but the total amount of Al3Zr phase reduces. The theoretical value of solid fraction agrees well with experimental value. With the decrease of pouring temperature, the relative content of thick-plate Al3Zr phase increases in terms of the solid fraction.