The effect of adding cellulose nanofibers (CNFs) to a silicon nitride slurry was evaluated. The addition of small amounts of CNFs drastically increased the viscosity of the silicon nitride slurry. Pouring the slurry with CNFs into a non-water absorptive mold, followed by natural drying, yielded a silicon nitride green body. Sintered silicon nitride was successfully fabricated via de-binder treatment and sintering of the green body. The sintered silicon nitride had a density of 3.142 × 10³ kg m⁻³, and a microstructure with rod-like grains and a glassy grain boundary phase. The flexural strength of the sintered body was 595 MPa. These properties were comparable to those of silicon nitride fabricated from a slurry without CNFs.