Yarns of different types are unwound from bobbins in different processes like warping, weaving, doubling and re-winding. A change in yarn tension from a full to empty bobbin causes serious product quality and machine efficiency problems in processes like weaving and winding. This paper investigates experimentally the relationship between yarn tension and bobbin diameter in the warping process. For this purpose, an experimental set up was built with a laser sensor measuring the bobbin diameter, a tension sensor for measuring yarn tension, a bobbin winding unit, PC, and DAQ cant A software program was developed in the C programming language to read and record the tension and bobbin diameter simultaneously. The unwinding speed and yarn number both had a significant effect on the relationship between the yam tension and bobbin diameter. The effect of the unwinding speed became more pronounced with yarns getting thicker.