MARS and Rijndael are analyzed by using power attack. It is shown that the complexities on MARS are 2²⁰⁸, 2¹⁶⁸ and 2¹¹⁶ for 256, 192 and 128 bits key respectively, the complexities on Rijndael are 2¹³¹, 2⁹⁹ and 267 for 256, 192 and 128 bits key respectively. Although the complexities are too big to implement, the approach presented in this paper reduces greatly the key size of MARS and Rijndeal.