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In this work, we used femtosecond‐laser direct writing to fabricate two kinds of modified structures with opposite refractive‐index changes to build fork‐grating configurations in z‐cut LiNbO3 (LN) crystals. Experimental and simulation results indicate that the fork grating combined with the modified area of the refractive index increased and refractive index decreased produces a more effective diffraction effect at 532 nm. The diffraction results of these fork gratings have been investigated. Compared with the fork grating only with the refractive‐index‐decreased filaments, when the incident beam is at transverse magnetic, transverse electric, and circular polarizations, the diffraction efficiency is improved by 5.3%, 4.3%, and 9.6%, respectively. Our work provides a new perspective for improving the diffraction efficiency of fork gratings in LN crystals. [ABSTRACT FROM AUTHOR]