The initiation sites and influencing factors of cavity nucleation were investigated for a Ti-6Al-2Zr-1Mo-1V alloy with lamellar starting structure, using the isothermal hot compression test. All samples were deformed to a true strain of 0.70 in the temperature range of 750-950 ℃ and strain rate range of 0.001-10 s-1. The corresponding microstructures were observed by means of the metallurgical microscopy and scanning electron microscopy (SEM). It was found that all cavities occurred at the bulge regions of the compression specimens. Most of cavities nucleated along prior beta boundaries oriented 45° to the compression axis, while others nucleated at the interfaces of lamellar alpha colonies. Cavity nucleation was inhibited with increasing the volume fraction of beta phase and the volume fraction spheroidized of lamellar alpha phase.