For high quality cotton spun yam, the drafting conditions in the ring spinning machine have to be determined considering the cohesive force of the roving which is related to its twist factor, count and constituent fiber properties. In this study we determine the break draft efficiency by measuring the rotational speed of middle top apron and back top roller in the break draft zone of a ring spinning machine, and investigate the relationship between break draft efficiency and the quality of cotton spun yam. The results show that the break draft efficiency decreases and the count of cotton spun yam becomes coarser with increase in break draft ratio when break draft ratio is less than the optimum value. When the break draft ratio is the optimum value, the break draft efficiency reaches a minimum and the quality of cotton spun yam is improved. When the break draft ratio is larger than the optimum value, the break draft efficiency increases sharply with increase in break draft ratio due to stick-slip phenomenon, the variation of break draft efficiency becomes larger and the quailty of cotton spun yam is negatively affected. It is also shown that the higher the cohesive force of roving, the higher the break draft ratio where break draft efficiency reaches a minimum.