PURPOSE: To obtain polyester fibers for reinforcing rubber which is produced by adding a specific di-epoxy compound to polyester and shows excellent adhesion to rubber, and excellent heat resistance and dimensional stability in the rubber.

CONSTITUTION: Polyethylene terephthalate is combined with 0.005 to 10wt.%, preferably 0.1 to 3wt.% of a di-epoxy compound of formula I (Ar is an aromatic group of 6 to 20 carbon atoms), then melt-spun through an extruder at a high speed. The undrawn yarn is drawn to give polyester fiber suitable for reinforcing rubber having less than 12% of the sum of the middle elongation and the dry heat shrinkage (177°C×30 minutes), less than 25eq/ton of carboxyl groups and less than 1.3wt.% of diehylene glycol.