Fracture toughness is a key parameter in determining the stability of interlaminar structures. The interface between the cemented paste backfill (CPB) and the adjacent rock mass, which is one of the key controlling factors of the mechanical stability of a CPB structure, is regarded as an interlaminar structure. Fractures (mode-I, mode-II) can occur and propagate along this interface, thereby jeopardizing the stability of the CPB structure. In other words, assessing the fracture toughness of CPB-rock interface is essential for the stability analysis of CPB structures. Although many studies have been conducted on the fracture toughness of brittle materials, the evolution of the modes I & II fracture toughness of the interface between the cemented paste backfill (CPB) material and rock still remains unknown. Given the significant effect of curing time, temperature and sulphate ions on the binder hydration, it is necessary to study the evolution of the modes I & II fracture toughness of the CPB/rock interface under different thermal and chemical conditions. Therefore, many semi-circular samples (with or without sulphate ions) were produced and cured at 2 °C, 20 °C, and 35 °C for 3 days, 7, 14 and 28 days in this study. Thereafter, all specimens were subjected to semi-circular bend (SCB) tests. The obtained results show that both temperature and sulphate ions have considerable effects on the evolution of the modes I & II fracture toughness. Generally, the modes I & II fracture toughness rises as the temperature is increased (with an exception of the samples cured at 35 °C for 28 days). Nevertheless, the influences of sulphate ions on the growth of the modes I & II fracture toughness vary with the age of samples and the initial contents of sulphate ions.