In literature, it has been shown that Fast Frequency Hopping Orthogonal Frequency Division Multiple Access (FFH-OFDM) exploits frequency diversity whereas classical OFDM does not. In this work, a semi-analytical expression for the symbol error probability (SEP) of FFH-OFDM with Minimum Mean Square Error (MMSE) equalizer over flat Rayleigh channels is derived, and validated with simulations. In addition, FFH and classical OFDM performances are compared by using the derived expressions. Numerical results show that FFH-OFDM outperforms Classical-OFDM in low modulation levels due to its frequency diversity.