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We reported, for the first time, the fracture toughness of aluminum nitride ceramics at cryogenic temperatures. The fracture toughness increased from 3.98±0.19 to 4.59±0.28 MPa m1/2 with the decreasing temperature from 293 to 77 K. Increasing fraction of transgranular fracture (from 7.3% at 293 K to 14.5% at 77 K) was proposed as a governing mechanism for toughness improvement at cryogenic temperatures based on the experimental observations. The effect of residual stresses on crack propagation was theoretically analyzed, considering energy release rates of intergranular and transgranular fractures. The variation of crack paths would contribute to the change of fracture modes and fracture toughness at cryogenic temperatures. [ABSTRACT FROM AUTHOR]