  The effects of regular S=1 dilution of S=1/2 isotropic antiferromagnetic
chain are investigated by the quantum Monte Carlo loop/cluster algorithm. Our
numerical results show that there are two kinds of ground-state phases which
alternate with the variation of $S^1=1$ concentration. When the effective spin
of a unit cell is half-integer, the ground state is ferrimagnetic with gapless
energy spectrum and the magnetism becomes weaker with decreasing of the $S^1$
concentration $\rho = 1/M$. While it is integer, a non-magnetic ground state
with gaped spectrum emerges and the gap gradually becomes narrowed as fitted by
a relation of $\Delta \approx 1.25\sqrt{\rho}$.
