The yarn is transferred from one bobbin to another in many textile processes. In these kinds of processes like warping, weaving, doubling and twisting, yarn tension change with respect to bobbin diameter has a significant role because it affects product quality and machine efficiency. Yarns including lycra component are widely used in fabric production which are sensible to tension changes. This paper presents an experimental research aiming at determining the relationship between yarn tension and bobbin diameter during unwinding for continuous filament polyester-lycra yarns. For this aim, an experimental research set-up was established representing unwinding from bobbins in many textile processes. Experimental research set-up consists of a winding machine, a creel with single bobbin holder, a laser sensor measuring bobbin diameter, a tension sensor for measuring yarn tension, a PC and a DAQ card. A software program was developed in C programming language to read and record the tension and bobbin diameter simultaneously as a data file. Based on the recorded data, average values of warp tension and bobbin diameter were calculated and relationship between yarn tension and bobbin diameter was obtained in this way. Experimental work was carried out for 3 different yarn numbers (75, 150 and 300 denier) at 5 different unwinding speeds (100, 200, 400, 600 and 800 m/min). The results were compared with the yarns without lycra component and found to be in agreement with theoretical findings in the literature expressing that high elasticity of yarns limited the tension change from full to empty bobbin.