Aiming at the plaintext recovery on plaintexts encrypted by RC4 (Rivest Cipher 4) algorithm with different lengths of seed key, a plaintext recovery attack on plaintexts encrypted by RC4 algorithm with different lengths of seed key (8 bytes, 16 bytes, 22 bytes) was proposed. Firstly, by using the statistical algorithm, the t-value distribution of each output byte of key stream of RC4 was calculated under the condition of 2 ³² different seed keys, and biases were found. Then the attack on the first 256 bytes of the plaintext encrypted by the RC4 was given by using single-byte biases and double-bytes biases. The experimental results show that with 2 ³¹ ciphertexts, the first 196 bytes of the plaintext can be recovered with the success probability of 100% except the 4th Byte. Besides, the first 256 bytes can be recovered with the success probability over 91%, 87% and 81% for 8-byte, 16-byte and 22-byte seed key, respectively. The proposed attack algorithm extends the scope of RC4 algorithm with seed key length of 16 bytes, and it can recover the plaintexts encrypted by RC4 algorithm in practice.