To evaluate the security of a class of generalized Feistel ciphers

the security evaluation against differential and linear cryptanalyses is investigated deeply using iterative structure.If round functions are all bijective

then the number of active round functions for 4r-round (r1) ciphers are not less than (8/3)r-[(rmod3)/3]+(rmod3)/3.The result is at least improved 20% than the existing result when r6.So the upper bounds of maximum differential characteristic and linear approximation probabilities for 4r-round (r1) ciphers can be estimated if maximum differential and linear approximation probabilities for round function are given.