  To reduce the effect of the fluctuations of initial geometrical shape,
elliptic flow are studied at fixed centrality in Au+Au collision at
$\sqrt{s_{\rm NN}}$=200GeV. It is observed with the participant increasing,
elliptic flow has an increase and a decrease at different fixed impact
parameter, but not a trivial fluctuation. It is analyzed that the initial space
anisotropy dominates the participant dependence of elliptic flow in
near-central collisions(b=5fm) and mid-central collisions(b=8fm), while the
interaction between particles can mainly answer for the behavior of elliptic
flow in peripheral collisions(b=12fm). To distinguish the pure geometrical
effect, elliptic flow scaled by initial eccentricity is studied. It is found
that the ratio $v_{2}/\epsilon$ increases with participant and reaches a
saturation when the participant is large enough, indicating that the collision
system reach the local equilibrium.
