In order to obtain excellent comprehensive mechanical properties, it is important to control the lamellar orientation of TiAl-based alloys by directional solidification technology. The control of the lamellar orientation is related closely with the primary phase during the directional solidification of TiAl-based alloys. The identification of the primary phase in directionally solidified TiAl-based alloys is insufficient only according to an isolated two-dimensional micrograph of the lamellar orientation. In this paper, we identified accurately the primary phase according to the quasi three-dimensional micrograph of the lamellar microstructure. The results indicate that the primary phase with various lamellar orientations is β phase during the directional solidification of Ti-50Al-5Nb alloy