Room temperature fatigue of ZrB₂-30 wt% SiC ceramic composite has been studied. The linear regression technique, proposed by Fett and Munz, was used to determine the static and cyclic fatigue parameters of ZrB₂-30 wt% SiC ceramic composite tested in dry air, air with 40% humidity, and water at room temperature. It was found that cracks grow much faster under cyclic test conditions in comparison with constant static loading. This phenomenon can be attributed to (1) the degradation of bridging interactions between the two growing crack surfaces; and (2) a corresponding decrease in shielding stress intensity factor. When cyclic fatigue experiments were performed in water in comparison with the results obtained in air, it was also found that crack grows much faster under 100% of humidity. It was determined that water molecules increase the crack propagation rate in ZrB₂-30 wt% SiC composite. The lifetime of the material decreases from 18 hours at σ = 580 MPa static loading in air to 5.5 minutes at Δσ = 1200 MPa cyclic loading in air and 0.4 seconds at Δσ = 1240 MPa cyclic loading in water.