  The Rossiter-McLaughlin (RM) effect has been widely used to estimate the
sky-projected spin-orbit angle, $\lambda$, of transiting planetary systems.
Most of the previous analysis assume that the host stars are rigid rotators in
which the amplitude of the RM velocity anomaly is proportional to $v_\star \sin
i_\star$. When their latitudinal differential rotation is taken into account,
one can break the degeneracy, and determine separately the equatorial rotation
velocity $v_\star$ and the inclination $i_{\star}$ of the host star. We derive
a fully analytic approximate formula for the RM effect adopting a parameterized
model for the stellar differential rotation. For those stars that exhibit the
differential rotation similar to that of the Sun, the corresponding RM velocity
modulation amounts to several m/s. We conclude that the latitudinal
differential rotation offers a method to estimate $i_\star$, and thus the full
spin-orbit angle $\psi$, from the RM data analysis alone.
