The propagation behavior of machining-induced cracks in silicon nitride was investigated by conducting constant-stress and constant-stress-rate tests. A dye-impregnation technique that forced a palladium nitrate solution into the cracks gave clear evidence of subcritical crack growth of machining cracks in suspended constant-stress test specimens. The fracture origin before subcritical crack growth usually consisted of a series of small cracks within an elongated semielliptical envelope. The dye-impregnation results gave evidence that some of the elemental cracks had coalesced during subcritical crack growth. It was hypothesized that coalescence might explain an initially higher crack-growth rate suggested by constant-stress-rate tests.