Abstract
A set of equations that allows one to calculate the correlation length and the free energy of the S=1/2 XYZ chain at a given temperature is obtained. It contains infinite unknown numbers and is derived by the largest and the second-largest eigenvalues of the quantum transfer matrix in the limit of infinite Trotter number. The numerical solution of this set of equations gives very accurate values of the free energy and the correlation length at arbitrary temperature. The energy gaps that appear in the correlation length and the free energy at low temperature are discussed.