Isopogon anethifolius is a shrub usually between 1 and 3 m (3 and 10 ft) high with an upright habit (tall and thin with mostly vertical stems). It generally grows taller in more sheltered areas such as woodlands, and shorter in more exposed areas. The stems are reddish in colour, and new growth in winter is tinged with reddish and tan tones.The leaves are terete (round in cross section) and less than 1 mm (1⁄25 in) in diameter. They branch once or twice in their 16 cm (6 1⁄4 in) length. The globular yellow flowerheads, known as inflorescences, appear at the ends of branches in spring and early summer (September to December), though occasionally at other times of year.These are up to 4 cm (1 1⁄2 in) in diameter. The individual flowers arise out of the central woody globe in a spiral pattern, and are around 1.2 cm (1⁄2 in) long. They are straight stalkless structures that originate from a scale on the globe, composed of a tubular structure known as the perianth, which envelopes the flower's sexual organs. The perianth splits into four segments, revealing a thin delicate style tipped with the stigma. At the ends of the four perianth segments are the male pollen-bearing structures known as anthers. Arranged in a spiral pattern, the flowers open from the outer/bottom of the flowerhead inwards. The egg-shaped grey cones are revealed as the old flower parts fall away, and are up to 2.5 cm (1 in) in diameter. The seed-bearing nuts are small—up to 4 mm (3⁄16 in) across—and lined with hairs. The seed weighs around 4 mg (0.00014 oz).
The terete leaves readily distinguish Isopogon anethifolius from other members of the genus, which have flat leaves and are greater than 1 mm (1⁄25 in) across. On a microscopic level, the supporting ground tissue of I. anethifolius differs from some of its genus by its irregular misshapen sclereids (thick-walled cells that make up part of the ground tissue) and contorted cell body