Purpose Yarns of different types are unwound from bobbins in different processes like warping, weaving, doubling and re-winding. It is required that yarn tension remains constant during unwinding in all these processes. Otherwise, it ends with product quality and process efficiency problems. The purpose of this paper is to investigate experimentally the effect of balloon length on yarn tension change with respect to bobbin diameter during unwinding in an attempt to obtain a minimum yarn tension variation. Design/methodology/approach An experimental set up was built. Bobbin diameter was measured by a laser sensor and yarn tension was measured by a single yarn tension sensor. Both sensor outputs were interfaced to a PC via a DAQ cad. A software program was developed in C programming language to read and record the tension and bobbin diameter simultaneously. Experimental study was conducted with three different balloon lengths for both continuous filament and spun yarns of four different yarn numbers and five different unwinding speeds. Findings Results showed that yarn tension change with bobbin diameter was affected in different ways with balloon length depending on yarn number, unwinding speed and yarn type. Originality/value Available literature on the effect of balloon length on yarn tension bobbin diameter relation is limited and measurements were generally conducted for three different bobbin diameters. Yarn tension bobbin diameter relation is obtained in this research for at least eight different diameters and more for three different balloon lengths covering practical application ranges. The results obtained can be used in the design of tension control system for warping and winding machines as well as for setting these machines for optimum efficiency.