An improved impossible differential attack on the block cipher CLEFIA is presented. CLEFIA was proposed by Sony Corporation at FSE 2007. Combining some observations with new tricks, the wrong keys are filtered out more efficiently, and the original impossible differential attack on 11-round CLEFIA-192/256 published by the designers, is extended to CLEFIA-128/192/256, with about 2^103.1 encryptions and 2^103.1 chosen plaintexts. By putting more constraint conditions on plaintext pairs, we present an attack on 12-round CLEFIA for all three key lengths with 2^119.1 encryptions and 2^119.1 chosen plaintexts. Moreover, a birthday sieve method is introduced to decrease the complexity of the precomputation. And an error about the time complexity evaluation in Tsunoo et al.’s attack on 12-round CLEFIA is pointed out and corrected.