In this paper, a mathematical analysis of warp elongation is carried out in weaving machines with positive backrest system. The influence of backrest swinging angle, shed closing angle, backrest orientation angle and backrest phase difference on warp elongation is investigated. This study is done for symmetrical shed and heddle motion both with and without dwell. Both backrest and heddle mechanisms are assumed to have simple harmonic motion curves. Results have shown that especially shed closing angle and backrest phase difference have an important effect on warp elongation curve over a loom revolution. As a result, it can be concluded that the requirements of weaving of different type of fabrics and loom settings cannot be met by a motion curve obtained by crank rocker or cam mechanism with positive backrest mechanisms. By producing different motion curves with motor driven positive backrest, optimum warp elongation or warp tension curves can be obtained for weaving of different type of fabrics.