Abstract: A process of marking fly fishing lines includes the steps of selecting a heat shrinkable tubing having predetermined shrinking temperature characteristics and marking the tubing with a size coding positioned to allow the marks to be cut off to indicate any of a range of fishing line sizes or weights. The tubing is then cut to form small sections with the marking thereon and may then further be cut to remove marks to indicate one size. The tubing is slipped onto a fly fishing line and immersed in a heated liquid, such as water, or water having salt added thereto, to shrink the tubing onto the fishing line without damaging the fishing line.