Serpent is an SP Network block cipher submitted to the AES competition and chosen as one of its five finalists. The security of Serpent is widely acknowledged, especially as the best known attack so far is a differential-linear attack on only 11 rounds out of the 32 rounds of the cipher. This paper proposed impossible differential-algebraic cryptanalysis of 11-round Serpent using the key arrangement scheme of Serpent as well as the attack principle of impossible differential analysis, according to the algebraic equation of S-box. The new method analyzed block 11-round Serpent with selected plaintexts and memory space.