  Nucleon sea structure functions are studied using
Dokshitzer-Gribov-Lipatov-Altarelli-Parisi (DGLAP) equations with the massive
gluon-quark splitting kernels for strange and charm quarks, the massless
gluon-quark splitting kernels for up and down quarks, and the massless kernels
for all other splitting parts. The $SU(2)_f$ flavor symmetry for two light
quarks, `up' and `down', is assumed. Gl\"uck-Reya-Vogt(GRV) and
Martin-Roberts-Stirling(MRS) sets are chosen to be the base structure functions
at $Q_0^2=3$GeV$^2$. We evolve the sea structure functions from $Q_0^2=3$
GeV$^2$ to $Q^2=50$ GeV$^2$ using the base structure function sets and DGLAP
equations. Some (about 10%) enhancement is found in the strange quark
distribution functions at low $x(<0.1)$ in leading order of the DGLAP equations
compared to results direclty from those structure function sets at the the
value of $Q^2=50$ GeV$^2$. We provide the value of $\kappa$ and also show the
behavior of $\kappa(x)=2s(x)/(\bar u(x) + \bar d(x))$ after the evolution of
structure functions.
