this paper aims to investigate experimentally the disengagement of needle thread from rotating hook in an industrial single-needle lockstitch sewing machine with respect to tightening tension. for this purpose, the disengagement of the thread from rotating hook was photographically observed, and the resultant changes in needle thread tension were recorded simultaneously. the experimental investigation revealed that the timing of the disengagement of needle thread from rotating hook is affected by the elongation properties of the sewing threads. if cotton thread low in elongation was used, the needle thread first slips out of the jib and thereafter slips out of the 'rotating hook bobbin case holder position bracket'. when polyester or nylon threads higher in elongation than cotton are used, disengagement of needle thread from the rotating hook is normal since the needle thread first slips out of the bracket and thereafter slips out of the jib. when the disengagement occurs in this order the timing of disengagement has no effect on the tightening tension. 4 refs