Usideroxylon zwageri or Ulin wood is a slow growing (0.5 metres per year) tall evergreen tree with a straight bole (usually host to Cassytha, a parasitic vine with leaves reduced to scales, up to half of the tree's height). It is slightly fluted at the base, up to 150–220 cm in diameter. The trunk has many small, rounded buttresses that give the base an elephant-foot like appearance. Individual trees may reach an age of 1,000 years or more. Common commercially exploitable trees attain a height of 30 or more metres (100 ft) with trunk diameters of exploitable trees up to 92 cm (36 inches). Protected trees are towering giants of the forest attaining a height of up to 50 metres and a diameter of 220 cm – though height is routinely reduced by lightning strikes. An Ulin tree discovered in 1993 in Kutai National Park, is one of the largest plants in Indonesia. It is an estimated 1,000 years old, and has increased its diameter from 2.41 to 2.47 metres in the 20 years since its discovery. Its height was however reduced from some 30 metres to only 20 after a lightning strike. Another at Sangkimah in the west of the park has a diameter of 2.25 metres and a height of some 45 metres.
The trees' leaves are dark green, simple, leathery, elliptical to ovate, 14–18 cm long (5.5–7.5 inches) and 5–11 cm wide (2–4 inches), and are alternate, rarely whorled or opposite, without stipules and petiolate. The leaf blade is entire (unlobed or lobed in Sassafras) and occasionally with domatia (crevices or hollows serving as lodging for mites) in axils of main lateral veins (present in Cinnamomum).