The present paper presents the fracture toughness of carbon fiber reinforced plastic (CFRP) bonded joints at cryogenic temperature. In order to investigate mode I and mode II fracture toughnesses, double cantilever beam (DCB) and three-point end notched flexure (ENF) tests are conducted on CFRP/adhesive/CFRP, CFRP/adhesive/aluminum plate bonded specimens at room temperature (296 K), at low temperature (223 K) and at cryogenic temperature (77 K). Effect of thermal deformation on the fracture toughness is considered since large difference in coefficients of thermal expansion (CTE) between CFRP and aluminum alloy generates considerable thermal stress at cryogenic temperature. In both DCB and ENF tests, crack extends in the adhesive film (cohesive failure) at room temperature, whereas it propagates along the interface between CFRP and adhesive film (interfacial failure) at cryogenic temperature. The fracture toughness becomes lower when interfacial failure occurs. DCB and ENF tests results reveal that the transition of the failure mode from the cohesive failure to the interfacial failure significantly degrades the fracture toughness of bonded joint, and that the transition occurs more easily at lower temperature. (C) Koninklijke Brill NV, Leiden, 2012