The quality of woven fabrics is deeply affected by the regularity of warp and weft density, which is one of the fundarnental fabric pararneters determining the physical properties of fabrics. In particular the regularity of weft density can be easily destroyed in a transient or unstable processing state, for example: when weaving process is interrupted by a disturbance or in the case of article change. In most cases, a reruning of the weaving machine leaves a fabric fault in the form of streaks which caused by the irregularity of weft density. This fault is called "stop mark" and is nowadays one of the most frequently occurring errors in fabrics. In this study, it was tried to analyze the mechanism how a stop mark takes place and to obtain a theoretical model. During modelling, it was proposed that the latest pickspacing can be affected by the beating motion which build up a new pickspacing as a result of new weft insertion. The mathematical model has shown a nonlinear simultaneous difference equation system which consists of equations about the pickspacing and clothfell position. The results of simulation revealed that forward movement of dothfell position has a sensitive effect on occurrence of dense weft streaks. The backward movement of clothfell position, however, brought about a broad and thick weft streaks.t streaks.