To provide polyester monofilament for screen gauze, which is excellent in durability and printing accuracy, good in weavability.

The polyester monofilament for screen gauze is sheath/core conjugate monofilament with the core component having polyethylene naphthalate as the main component and the sheath component having polyethylene terephthalate as the main component, wherein the weight ratio of core component to sheath component is (80:20) to (40:60). The polyester monofilament simultaneously meets the following requirements(1) to (4): (1) knot tenacity3.0 cN/dtex, (2) loop tenacity2.5 cN/dtex, (3) tensile strength4.5 cN/dtex, and (4) elongation at break10%.