In this paper, using finite element simulations and experimental results, the changes in deformation length and longitudinal strain in flexible roll forming are investigated and the relationship between them is determined. Flexible roll forming is a novel manufacturing process for producing profiles with variable cross-section. One of the important parameters of this process is the distance from the starting point of the deformation before the forming station to the central cross-section of the rolls at that station, which is called the deformation length. This parameter plays a key role in determining the distance between the forming stations and the deformation behavior of the sheet. The effect of roll diameter and mechanical properties of the sheet on the deformation length is also determined. The results showed that the maximum deformation length occurs when forming the stretching zone of the channel profile with variable cross-section, which is due to the additional tension applied to the edge due to the concave geometry of the flange in this zone. The results also showed that with increasing roll diameter and yield stress, the deformation length in all four of the stretching and compression zones and the slim and wide areas of the channel profile with variable cross-section increases, while with increasing sheet thickness, the deformation length in these zones decreases.