  In this article, we mainly investigate the properties of vertical velocity v
for two dimensional steady water waves over a flat bed. Firstly we prove the
existence of the inflection point for each streamline, then we find the
behavior of v along each streamline depends strictly on concavity and convexity
of streamline, which contributes to complete Constantin conjecture on v in
Stokes wave. And the location of maximum vertical fluid velocity is also proven
to be at the inflection point. Besides, we also extend our results to the cases
with monotonous vorticity.
