  We calculate the values of the second moment $M_2^s$ of the flavor-singlet
structure function $F_2^s$ for the pion and the kaon in QCD sum rules, and
investigate how the results depend on their flavor structure (quark contents).
Our calculations give similar values of $M_2^s$ for these two mesons, because
of cancellation among several non-small factors. We emphasize that decay
constants, meson masses and quark masses play an essential role in the above
cancellation.
