The theory of the meshing limit line of a ZC1 worm pair is fully established on the basis of meshing theory for gear drives. A method to judge the existence of the meshing limit line is proposed. Meanwhile such a method can provide the reasonable initial value for iteratively solving the nonlinear equation encountered during determining the meshing limit line. The numerical example is provided for validation and verification and the result shows, usually, the working length of a ZC1 worm cannot be in excess of the half of its thread length.