PURPOSE: To provide a polyester thick and thin yarn capable of being dyed into a deep black color suitable for black formal suits by subjecting polyester highly oriented undrawn yarn containing black carbon fine particles to a thermal shrinkage treatment and subsequently drawing the yarn.

CONSTITUTION: Polyester thick and thin yarn comprising polyester multifilaments, having a residual elongation of ≥40% and having a thick part/ thin part ratio of ≥1.1 among single filaments constituting the multifilaments, the multifilaments being produced by subjecting polyester highly oriented undrawn filaments containing 0.1-10wt.% of black carbon fine particles to a thermal shrinkage treatment by the use of a thermal treatment device, etc., and subsequently drawing the treated filaments in a drawing ratio largely not exceeding the thermal shrinkage amount. Polyester false-twisted and shrunk yarn comprising the above-mentioned polyester thick and thin yarn and having a thick part-occupying rate of 1-50% among the single filaments constituting the multifilaments in the longitudinal direction, and having a difference of ≥5% between the filament lengths of the multifilaments.