In order to develop ultra precision master gears, this paper takes the Y7125 grinder as an example to analyze the effect of the cone-shaped error of grinding face on the helix deviations.It establishes a geometric model and derives the numeric expression of the effect.The theoretical results show that the minimum effective bearing width has a little effect on the form deviation of gear helix.However, as the increase of gear width, the effect of the cone-shaped error on the form deviation of gear helix will significantly increase.Moreover, the cone-shaped error can make the curve of gear helix deviation to be an arc-shape and the crowns of arc vary from the dedendum to the addendum.Finally, the error measurement, gear-grinding experiment and deviation analysis are conducted by an example.The results show that the ultra-precision gear can achieve the quality of 1 helix deviation by controlling the cone-shaped error of grinding face of flat faced wheel under 2.7′.