Protein tyrosine nitration is a common post-translational modification occurring under conditions of nitrative/oxidative stress in a number of diseases. The major pathways of protein tyrosine nitration in vivo include peroxynitrite (ONOO(- )) and hemoglobin/[image omitted] /H(2)O(2)-dependent reaction. In this paper, several structural diversity flavonoids (quercetin, kaempferol, (+)-catechin, baicalein, apigenin, and naringenin) were chosen, to study their efficiencies against ONOO(- ) or hemoglobin/NaNO(2)/H(2)O(2)-mediated nitrative/oxidative damage to human plasma proteins in vitro. Protein nitration was efficiently inhibited by these flavonoids regardless of nitration pathways, and the inhibitory effects were consistent with their free radical scavenging activities. These flavonoids dose dependently inhibited ONOO(- )-induced protein oxidation, while they ineffectively suppressed hemoglobin/NaNO(2)/H(2)O(2)-triggered protein oxidation. These results mean that ONOO(- ) and hemoglobin/NaNO(2)/H(2)O(2) can cause plasma protein nitrative and oxidative damage in different pathways, and those flavonoids with strong antioxidant activities may contribute their protective effect partly through inhibiting protein nitration.