  In this paper, a symmetry classification of a $(2+1)$-nonlinear wave equation
$u_{tt}-f(u)(u_{xx}+u_{yy})=0$ where $f(u)$ is a smooth function on $u$, using
Lie group method, is given. The basic infinitesimal method for calculating
symmetry groups is presented, and used to determine the general symmetry group
of this $(2+1)$-nonlinear wave equation.
