In a lockstitch sewing machine, influence of sewing speed on stitch formation is investigated for two different dial positions on feed-length scale. When the dial position, is fixed at 3 on the scale (static stitch length is 3.1mm), the location of interlacing point in stitch shifts toward the upper side and stitches become tightened with the increase of sewing speed. However, when the dial is fixed at 2 (static stitch length is 2.0mm), the interlacing point shifts toward the upper side for only up to 2, 000 spm where after it moves toward the lower side with increasing sewing speed. The phenomenon arising in the former scale position could be explained by the facts of dependencies of both tightening tension and feed length on needle and bobbin thread tightening ratios: increase of tightening tension causes reduction of needle thread length in a stitch and thus makes the stitch tight, while the increase of feed length makes bobbin thread length in a stitch longer which causes the shifting of interlacing point toward the upper side. The phenomenon for the scale position at 2, can be estimated that at a low speed the position of interlacing point varies due to tightening tension, but at a high speed it varies due to stitch length governed by a sewing speed variation. From this study, it could, therefore, be concluded that the folding situation of newer stitch by a presser foot must affect the stitch formation.