Factors influencing the product quality of the roller drafting operation can be referred to as the arrangement of the bundle fibers and the bundle thickness. However, the movement of individual fibers cannot be exactly controlled during the processing. The state of the bundle flow in the processing can be disturbed easily due to the change of the raw material or the process conditions, which can be attributed to the fact that the bundle flow dynamics is different from the continuum. Uncontrollable bundle fibers lead to the irregularity of the output bundle thickness. This study reports on the characteristics of the draft irregularity by simulation, when the system has the dynamic stability on the basis of the bundle flow dynamic model. Results reveal that the thickness variation of the drafted bundle depends on the model parameters; a uniform thickness or a periodical variation in thickness appears. The draft irregularity can take place more sensitive to the input bundle thickness than to the draft ratio in a limited range of the output bundle thickness variation. The thickness irregularity can commonly characterized by the fundamental frequency, even though the system has the parameters that assure it to be stable. The range of the irregularity frequency band, above which the thickness variation disappears, is dependent on the disturbing source, but the upper limit of the frequency range reads 5 times of the fundamental frequency that is defined by the transient characteristics of the bundle flow system.