Abstract: The reliability of TWTA is a central issue in the field of communication and broadcast satellite. In order to increase the reliability, the TWTA redundancy can be introduced. It can be shown that the cross-strapping redundancy is better than the redundancy individually for each channel. In this paper, the formule for the reliability of n:k "cold" cross-strappinp redundancy system is discussed, and that of n : k "warm" cross-strapping redundancy system and its approximete form when η≤0.1λ are prfsented.