  Let $M$ be a K\"ahler surface and $\Sigma$ be a closed symplectic surface
which is smoothly immersed in $M$. Let $\alpha$ be the K\"ahler angle of
$\Sigma$ in $M$. We first deduce the Euler-Lagrange equation of the functional
$L=\int_{\Sigma}\frac{1}{\cos\alpha}d\mu$ in the class of symplectic surfaces.
It is $\cos^3\alpha H=(J(J\nabla\cos\alpha)^\top)^\bot$, where $H$ is the mean
curvature vector of $\Sigma$ in $M$, $J$ is the complex structure compatible
with the K\"ahler form $\omega$ in $M$, which is an elliptic equation. We then
study the properties of the equation.
