OFDM (Orthogonal Frequency Division Multiplexing) is used for DTTB (Digital Terrestrial Television Broadcasting). It is not affected by delayed waves easily, so SFN (Single Frequency Network) is possible. However, the signal quality cannot be measured accurately with C/N (Carrier to Noise Ratio) or MER (Modulation Error Ratio) in SFN area because of delayed waves. In this paper, we propose a new evaluation method with "Margin" and "Environmental Degradation Index".