This article inquires theoretically into the transfer function of the servo draft system and into the influence of fiber length on draft control. Figure 1 illustrates the existing model roller system. Its differential equation is given by formula (1). The transfer function of the servo draft system is obtainable thus: ΔN1(s)=1/D(1-sT2/1-sT1)ΔN2(s)-N1/v1(1/1+sT1)ΔU1(s) This article discusses, with the use of Bode diagrams (Figs. 5, 6, 7, 8 and 9), the influence of fiber length on ΔN2(s) and ΔU1(s).