This research reports on the bundle drafting characteristics on the basis of a mathematical model describing the bundle flow dynamics. And the model parameters were experimentally measured or estimated. The kinematic viscosity of bundle was measured experimentally and the velocity variance parameter was estimated by comparing results from the experiment and from the simulation. The step response of the output bundle thickness to a step change in the draft ratio was applied. The kinematic viscosity of bundle was confirmed to be in the range between $0.5{ imes}10^6$ and $8.0{ imes}10^6cm^2/s$, which depends on the process conditions. As the draft ratio increases, the viscosity decreases dramatically in a low level of the draft ratio, but a draft ratio higher than 10 causes a very slow change in the viscosity, which is again affected by the roller setting. The kinematic viscosity remained, however, almost unchanged in the level of the roller setting longer than 45 mm. Under the draft ratio larger than 10, a high viscosity and a low velocity variance of constituent fibers had the effect to make the stability area narrower. Therefore the analysis results of bundle flow stability indicate that the draft ratio should be adjusted as small as possible in the high draft ratio level larger than 10 and the entanglement of the bundle be minimized for a large value of the velocity variance parameter, if the bundle flow system is maintained in a dynamic stable state.