There have been many attempts to define the notion of quasilocal mass for a spacelike 2-surface
in spacetime by the Hamilton-Jacobi analysis. The essential difficulty in this approach is to identify
the right choice of the background configuration to be subtracted from the physical Hamiltonian.
Quasilocal mass should be nonnegative for surfaces in general spacetime and zero for surfaces in
flat spacetime. In this letter, we propose a new definition of gauge-independent quasilocal mass
and prove that it has the desired properties.