  We theoretically investigate the spin diffusion coefficient tensor in the A
phase of liquid 3He in term of quasiparticle life-time by using the Kubo
formula approach at low temperatures. In general, the coefficient is a fourth
rank tensor for the anisotropic states and can be defined as a function of both
normal component of spin-current and magnetization. The quasiparticle life-time
is obtained by using the Boltzmann equation. We find that components of the
spin diffusion coefficient are proportional to T^(-2) at low temperatures. The
normal components of spin current, hence, are strongly diffusive and one can
ignore the contribution of these components to the stability of half quantum
vortices (HQV s) in the equal-spin-pairing of 3He-A state. Hence to make a
stable HQV, it is enough for one to consider weak interaction plus the effects
of Landau Fermi liquid.
