  Based on the linearized Eilenberger equations, the upper critical field
$(H_{c2})$ of mixed d- and s-wave superconductors has been microscopically
studied with an emphasis on the competing effects of mass anisotropy and spin
Zeeman coupling. We find the mass anisotropy always enhance $H_{c2}$ while the
Zeeman interaction suppresses $H_{c2}$. As required by the thermodynamics, we
find $H_{c2}$ is saturated at zero temperature. We compare the theoretical
calculations with recent experimental data of
YBa$_{2}$Cu$_{3}$O$_{7-+AFw-delta}$.
