  We present results for the hyperon vector form factor f_1 for $\Xi^0
\rightarrow \Sigma^+ l\bar{\nu}$ and $\Sigma^- \rightarrow n l\bar{\nu}$
semileptonic decays from dynamical lattice QCD with domain-wall quarks.
Simulations are performed on the 2+1 flavor gauge configurations generated by
the RBC and UKQCD Collaborations with a lattice cutoff of 1/a = 1.7 GeV. Our
preliminary results, which are calculated at the lightest sea quark mass (pion
mass down to approximately 330 MeV), show that a sign of the second-order
correction of SU(3) breaking on hyperon vector coupling f_1(0) is likely
negative.
