The modal parameters of a torsionally coupled shear building are determined from Green's functions by a two-step determination process. At the first step, the natural frequencies and damping ratios are identified by genetic algorithms before reliable values are selected using the property of the Green's functions that the two parameters are dependent only on the modal order. The damping ratios are revealed to be less than 3.5 % for all the four modes. At the second step, the participation vectors are first identified by genetic algorithms, restricting to narrow search ranges for the natural frequencies and damping ratios. Reliable values of the participation vectors are then selected using another property of the Green's functions. This results in the participation vectors with predominant spectrum tending to be determined well.