This paper introduces a kind of step motion mechanism of cam chain with coaxial input and output axes
studies its transmission functions from zeroth to second order
presents the geometry conditions that make the output of the driven sprocket wheel as a constant transmission ratio in the middle part of step angle and the design method of mechanismic dimensions with given step angle.The design example has shown that both the design method and monufacture of the step motion mechanism are simple and the transmission is steady.