A cascaded encryption scheme using physical chaos is realized primarily, which combines chaotic encryption with data encryption standard. The difference of statistical properties of cipher text is studied for two cases. One case is that the ciphertexts come from different schemes, i.e. the cascaded and the single-stage ones, respectively. The other is that the ciphertexts are generated by chaotic signals with different unpredictability in the same scheme. Because physical chaos is adopted in this system, there is no assured relationship between plaintexts and ciphertexts, and the statistical characteristics of ciphertexts should be better than （or about equal to） that of any other encryption system. The experimental results support this conclusion, and show that the stronger unpredictability of chaotic signals causes the weaker correlation of ciphertexts.