To provide a thick and thin yarn suitable as clothes and affording a fabric having softness, a fullness feeling, drapeability and moderate stiffness and firmness and a more natural feeling and to provide a method for production by which the yarn can efficiently be produced.

The thick and thin yarn has thickness unevenness in the longitudinal direction and ≥10% coefficient of variation of shrinkage percentage in boiling water. The form of the thickness unevenness has the thickness unevenness in the longitudinal direction of fibers with an evenness tester (yarn evenness tester) and the form of the thickness unevenness has a Normal waveform measured with the evenness tester (yarn evenness tester) satisfies the following conditions (1) to (3). (1) Peak parts having ≥3 peak points are present in a number of 1-4 based on 1 m in the longitudinal direction of the fiber and (2) the fineness ratio of the maximum peak point of the peak parts and the valley part is ≥1.1. (3) The fineness ratio of other peaks other than the maximum peak point of the peak parts to the valley part is ≥1.05.