Abstract. In this paper, the two dimensional Euler flow under a simple symmetry condition with hyperbolic structure in a unit square D = {(x 1 , x 2 ) : 0 < x 1 + x 2 < √ 2, 0 < −x 1 + x 2 < √ 2} is considered. It is shown that the Lipschitz estimate of the vorticity on the boundary is at most single exponential growth near the stagnation point.