  Phosphorus-doped silicon single crystals with 0.19 % <= f <= 99.2 %, where f
is the concentration of 29^Si isotopes, are measured at 8 K using a pulsed
electron spin resonance technique, thereby the effect of environmental 29^Si
nuclear spins on the donor electron spin is systematically studied. The
linewidth as a function of f shows a good agreement with theoretical analysis.
We also report the phase memory time T_M of the donor electron spin dependent
on both f and the crystal axis relative to the external magnetic field.
