Let v(x, t) = v r e r + v θ e θ + v z e z be a solution to the three-dimensional incompressible axiallysymmetric Navier-Stokes equations. Denote by b = v r e r + v z e z the radial-axial vector field. Under a general scaling invariant condition on b, we prove that the quantity Γ = rv θ is Hölder continuous at r = 0, t = 0. As an application, we prove that the ancient weak solutions of axi-symmetric Navier-Stokes equations must be zero (which was raised by Koch, Nadirashvili, Seregin and Sverak (2009) in [15] and Seregin and Sverak (2009) in [26] as a conjecture) under the condition that b ∈ L ∞ ([0, T ], BMO −1 ). As another application, we prove that if b ∈ L ∞ ([0, T ], BMO −1 ), then v is regular.