@ A thread feed mechanism in a sewing machine for paying-out thread in a predetermined amount necessary for forming one seam. The thread feed mechanism comprises a first signal generation device for producing feed amount and needle bar amplitude signals, a second signal generation device for producing a workpiece thickness signal, first means for adding square values of feed amount and amplitude signals, second means for multiplying a square root of the first means by a coefficient, third means for producing a signal varying depending upon the operation mode thereof, fourth means for multiplying an workpiece thickness signal by the output of the third means, and fifth means for adding the outputs of the second, third and fourth means.