Two-dimensional (2D) graphite carbon nitride (g-C3N4) nanosheets have been successfully used as a saturable absorber (SA) in a passively Q-switched Nd:LLF laser at 1.3 μm for the first time, to the best of our knowledge. Under an incident pump power of 9.97 W, the shortest pulse duration of 275 ns was acquired with output power of 0.96 W and pulse repetition rate of 154 kHz, resulting in a pulse energy of 6.2 μJ. In addition, the saturable absorption behaviors of zero-dimensional 12 nm g-C3N4 nanoparticles (g-C3N4-NPs) and three-dimensional ordered mesoporous g-C3N4 (mpg-C3N4) were also observed, although their morphology and structure were quite different from 2D g-C3N4. The experimental results introduce the potential application of g-C3N4 nanomaterials as SAs in Q-switched lasers.