We present a general analytical theory for unidirectional mode conversion in a periodic waveguide at the sub-wavelength scale. According to this theory, two methods are proposed to achieve the unidirectional mode conversion. One method is to make the spectrum of the permittivity variation function asymmetrically, and the other is to employ the tensor permittivity and realize the non-reciprocal overlap of the coupled modes. Based on these two methods, five unidirectional mode converters are designed and demonstrated. The presented theory provides guidance for design of unidirectional devices in eliminating integrated backscattering, optical circulation and complex communication topology construction.