Abstract Metal-organic framework glasses feature unique thermal, structural, and chemical properties compared to traditional metallic, organic, and oxide glasses. So far, there is a lack of knowledge of their mechanical properties, especially toughness and strength, owing to the challenge in preparing large bulk glass samples for mechanical testing. However, a recently developed melting method enables fabrication of large bulk glass samples (>25 mm 3 ) from zeolitic imidazolate frameworks. Here, fracture toughness ( K Ic ) of a representative glass, namely ZIF-62 glass (Zn(C 3 H 3 N 2 ) 1.75 (C 7 H 5 N 2 ) 0.25 ), is measured using single-edge precracked beam method and simulated using reactive molecular dynamics. K Ic is determined to be ~0.1 MPa m 0.5 , which is even lower than that of brittle oxide glasses due to the preferential breakage of the weak coordinative bonds (Zn-N). The glass is found to exhibit an anomalous brittle-to-ductile transition behavior, considering its low fracture surface energy despite similar Poisson’s ratio to that of many ductile metallic and organic glasses.