Glass-forming ability (GFA) is the ability of a liquid to avoid crystallization during cooling. Metal-organic frameworks (MOFs) are a new class of glass formers (1-3), with hitherto unknown dynamic and thermodynamic properties. We report the discovery of a new series of tetrahedral glass systems, zeolitic imidazolate framework-62 (ZIF-62) [Zn(Im 2-xbIm x)], which have ultrahigh GFA, superior to any other known glass formers. This ultrahigh GFA is evidenced by a high viscosity h (10 5 Pa·s) at the melting temperature T m, a large crystal-glass network density deficit (δP/P g)network, no crystallization in supercooled region on laboratory time scales, a low fragility (m = 23), an extremely high Poisson's ratio (v = 0.45), and the highest T g/T m ratio (0.84) ever reported. T m and T g both increase with benzimidazolate (bIm) content but retain the same ultrahigh T g/T m ratio, owing to high steric hindrance and frustrated network dynamics and also to the unusually low enthalpy and entropy typical of the soft and flexible nature of MOFs. On the basis of these versatile properties, we explain the exceptional GFA of the ZIF-62 system.