In order to evaluate statistical characteristics of both static and cyclic fatigue strength in HIP and normal sintered silicon nitrides, namely toughened Si₃N₄ and SN1, load, P-increasing (quasi-static loading) test and load range, ΔP-increasing (cyclic loading) test were conducted in the room air. Acoustic emission (AE) method was attempted during the fatigue tests for the toughened Si₃N₄. The SN1 exhibited static fatigue behavior, while the static strength of the toughened Si₃N₄ was independent of time, i.e., no static fatigue. However, cyclic fatigue behavior appeared in the both materials. ΔP-increasing tests with different initial value of maximum stress, σ_{max, i}, enabled to evaluate cyclic loading condition under which fatigue damage nucleated. The result suggested that σ_{max, i} influenced on the maximum stress at failure, σ_{max, f}, for statistically weak specimens in the both materials, and thus fatigue damage was nucleated at lower stress levels for these specimens. Although this behavior could not be confirmed by AE analysis, AE waveform simulation revealed that the crack volume nucleated under the cyclic loading were smaller than those under quasi-static loading. Based on the above results, observation of fracture surfaces and finite element analysis, the fracture mechanisms of static and cyclic fatigue were discussed in detail.