Forming limit diagram of aluminum AA7075 and titanium Ti-6Al-4V alloys was examined. For this aim, various strain paths from uniaxial to equi-biaxial tensions were considered. It was established that titanium alloy stretching potential is more than of aluminum alloy due to higher average strain hardening exponent. Also, the average anisotropy factor of AA7075 is about five times lower than that of Ti-6Al-4V resulting in the reduction of resistance to thinning during sheet metal forming processes. Furthermore, employment of BBC2000 yield criterion for AA7075 and Hill’s 1993 for Ti-6Al-4V in determining numerical forming limit diagrams resulted in a closer coincidence with the experimentations. Finally, imperfection factors of 0.970 and 0.955 for AA7075, and 0.935 and 0.960 for Ti-6Al-4V are respectively attained by yield surfaces of BBC2000 and Hill’s 1993.