We have substantiated a simple yet efficient, competitive, and practical method to automatically fabricate liquid crystalline grating cells with homogeneous or twisted nematic (TN) structures by one-step ultraviolet holographic exposure on an empty glass cell coated with a photocrosslinkable polymer liquid crystal with 4-(4-methoxycinnamoyloxy)biphenyl side group (P6CB) films. The polarization diffraction properties in the resultant liquid crystalline grating cells have also been investigated extensively by varying the grating pitch. The theoretical considerations on the basis of elastic continuum theory revealed that accumulation of elastic free-energy density due to the TN structure affected the diffraction properties, although the strong anchoring of our photoalignment material contributed effectively to form the expected spatial distribution of liquid crystalline directors in the grating cells.