Abstract: Through the analysis of the characteristics of the nuclear cross section covariance matrix, the method of transforming the nuclear cross section covariance matrix in multi-group form into the users’ group structures has been studied and a general code T-COCCO based on the method has been developed. The nuclear cross section covariance matrix built-in SCALE6.1 is applied and the nuclear cross section covariance matrix of some nuclides such as ²³⁵U, ²³⁸U, and ²³⁹Pu in 44 groups structures has been expended or collapsed into different users defined group structures, such as 33 groups, 47 groups and 70 groups. At the same time, the corresponding covariance matrix has been produced by using NJOY for comparison and the matrix eigenvalue and rank has been also studied for verification. The analysis indicates that the method studied in this paper is reasonable and T-COCCO code is convenient friendly and efficient for users to gain the desired multi-group nuclear cross section covariance matrix, which can be applied to uncertainty and sensitivity analysis of nuclear data and physical calculation.