PURPOSE: To obtain a polyester multifilament slub yarn having a mottle tone caused by the moderate bulkiness and the dyeability difference in the longitudinal direction of the yarn and a natural surface appearance caused by the difference of the shrinkability, having excellent antipilling performance and suitable for women's wear.

CONSTITUTION: This polyester multifilament slub yarn is composed of a copolymerized polyester composed of a dicarboxylic acid component, a glycol component and a phosphorus compound and containing thick parts having Δn of 20×10-3 to 80×10-3 and thin parts having An of 120×10-3 to 200×10-3 and randomly distributed in the longitudinal direction of the yarn. The ratio of cross-sections of the thick part to the thin part is 1.1-1.8, the knot strength is ≤1.5g/d and the knot elongation is ≤5% after dyeing.