Based on the results of total-energy calculations using full-potential linearized augmented plane wave method within the generalized gradient approximation (GGA),we determined the tetragonal fine structure of K(Ta0.56Nb0.44)O3，i.e.,the equilibrium configuration of B-site cations (Ta and Nb)in the ferroelectric phase. Ta is found to have an off-center displacement,relative to the oxygen octahedron,of about 0.005nm along [001]. The off-center displacement of Nb is as large as 0.016nm,thus makes a major contribution to the occurrence of the ferroelectric instability.