  Isoscaling behavior of the statistical emission fragments from the
equilibrated sources with $Z$ = 30 and $N$ = 30, 33, 36 and 39, resepectively,
is investigated in the framework of isospin dependent lattice gas model. The
dependences of isoscaling parameters $\alpha$ on source isospin asymmetry,
temperature and freeze-out density are studied and the "symmetry energy" is
deduced from isoscaling parameters. Results show that "symmetry energy"
$C_{sym}$ is insensitive to the change of temperature but follows the power-law
dependence on the freeze-out density $\rho$. The later gives $C_{sym}$ =
30$(\rho/\rho_0)^{0.62}$ if the suitable asymmetric nucleon-nucleon potential
is taken. The effect of strength of asymmetry of nucleon-nucleon interaction
potential on the density dependence of the "symmetry energy" is dicussed.
