  We study the isospin breaking effect on octet baryons. Using the two-flavor
dynamical domain-wall QCD configurations combined with the quenched non-compact
QED configurations, the electromagnetic mass splittings between isomultiplets
(p, n), (Sigma^+, Sigma^0, Sigma^-), (Xi^0, Xi^-) are investigated. We evaluate
the main source of statistical fluctuations in the two-point correlation
function, and find that the elimination of O(e) fluctuation (e: the QED charge)
is essential to extract the signal. Preliminary results for $m_p - m_n$ as well
as other mass splittings are presented. Possible origin of systematic
uncertainty is also discussed.
