As a tool of complex shape, the manufacturing error of hobs can be classed into three kinds: the datum level error, the tooth thickness error and the parameter error of helicoidal surface. The parameter error of helicoidal surface, which is composed of tooth error and helix error, is the most importance among them. For precision of hob and convenience of measurement, several individual errors are instead of the helix error. How to set reasonable individual errors to guarantee the helix error within tolerance, the relationship between them in gear hob measurements is analyzed, and the approach to decompose the helix error into individual error has been discussed in the paper. On the basis of above theory, a high precision mode can be easily established using the soft ware.