The critical fracture toughness J(1c) of the polycarbonate (PC)/acrylonitrile-butadiene-styrene (ABS) blend at different temperatures was obtained from ASTM E813-81, E813-87, and the recently developed hysteresis energy methods, respectively. The J(1c) value increases with increase of the test temperature ranging from -60 to 70 degrees C. The hysteresis energy method and the ASTM E813-81 method result in comparable J(1c) values, while the ASTM E813-87 results in about 80-110% higher values. The critical initiation displacements determined from the plots of hysteresis energy and the true crack growth length vs. crosshead displacement are very close. This indicates that the critical initiation displacement deter mined by the hysteresis method is indeed the displacement at the onset of true crack initiation and the corresponding J(1c) represents a physical event of crack initiation. The fracture toughness, K-1c value, based on linear elastic fracture mechanics (LEFM), was determined by using K-Q analysis (ASTM E399-78), and the obtained K-Q value decreases with the increase of the test temperature. The K-Q value is not the real LEFM K-1c value because the criterion of P-max/P-Q < 1.1 has not been satisfied. However, the corresponding J(Q) obtained from the K-Q analysis is comparable to the J(1c) obtained from the E813-81 method at lower temperature (-45 or -60 degrees C), an indication of LEFM behavior at lower temperature. The various schemes and size criterion based on LEFM and the J-method are explored for the validity of J(1c) and K-1c values.