The Cu-NMR spectrum for Cu(2) site was measured from 1.5 K to 200 K in the quasi-two dimensional (2D) bilayer antiferromagnet, YBa 2 Cu 3 O 6.1 . The nuclear Zeeman and quadrupole frequencies, ν L and ν Q were deduced from the spectrum. The decrease of the sublattice magnetization with increasing temperature was evaluated from the values ν L as (ν L (0)-ν L ( T ))/ν L (0) assuming no temperature dependence of the hyperfine coupling constant. The evaluated sublattice magnetization shows the almost linear temperature dependence at least above 25 K in log ((ν L (0)-ν L ( T ))/ν L (0)) vs. log T plot. This temperature dependence of the sublattice magnetization is well explained quantitatively within a factor of two by the spin wave model in the quasi-2D antiferromagnet using the exchange coupling parameters reported previously by neutron scattering experiments.