  The spreading of a cap-shaped spherical droplet on a completely wettable
spherical substrate is studied. The non-equilibrium thermodynamic formulation
is used to derive the thermodynamic driving force of spreading including the
line-tension effect. Then the energy balance approach is adopted to derive the
evolution equation of the spreading droplet. The time evolution of the contact
angle $\theta$ of a droplet obeys a power law $\theta \sim t^{-\alpha}$ with
the exponent $\alpha$, which is different from that derived from Tanner's law
on a flat substrate. Furthermore, the line tension must be positive to promote
complete wetting on a spherical substrate, while it must be negative on a flat
substrate.
