We have formed one-dimensional (1D) diffraction gratings on a photoresist polymer (SU-8) by interference exposure using a Lloyd mirror. A single crystal of thiophene/phenylene co-oligomers (TPCOs) grown in a vapor phase was placed on the 1D diffraction grating. Excitation of the single crystal by an ultraviolet light of a mercury lamp produced spectrally-narrowed fluorescence. On the basis of the grating periods and emission peak locations, we evaluated effective refractive indices and diffraction orders. The relevant information enables us to design and fabricate the 1D diffraction gratings that cause the spectrally-narrowed emission lines to superimpose inherently occurring high-intensity fluorescence peaks.