  Mass and isospin dependence of symmetry energy coefficients $a_{sym}$ of
finite nuclei is investigated with the measured nuclear masses incorporating
the liquid drop mass formula. The enhanced $a_{sym}$ for nearly symmetric
nuclei are observed. To describe the mass and isospin dependence of $a_{sym}$,
a modified formula based on the conventional surface-symmetry term is proposed
and the corresponding rms deviation of nuclear masses is checked.
