  We consider the effect of three species of neutrinos with nearly degenerate
mass on the cosmic structure formation in a low matter-density universe within
a hierarchical clustering scenario with a flat initial perturbation spectrum.
The matching condition for fluctuation powers at the COBE scale and at the
cluster scale leads to a strong upper limit on neutrino mass. For a flat
universe with matter density parameter Omega=0.3, we obtain m_nu<0.6eV for the
Hubble constant H_0<80km/s/Mpc. Allowing for the more generous parameter space
limited by Omega<0.4, H_0<80km/s/Mpc and age t_0>11.5Gyr, the limit is 0.9eV.
