The tension variation of warp yarns in the weaving process is one of the most important factors determining the irregularity of weft density in woven fabrics. Particularly, when the loom starts up, the tension of warp yarns varies greatly. So it is necessary to control this tension variation. In this research, to reduce the tension variation of warp yarns during start up, we adopted the active negative control of the let-off speed. First, the weaving mechanism was modeled, and the model was linearized to establish the system that controls the tension of warp yarns with let-off speed. Then, the tension of warp yarns was predicted by simulation. In simulation, the take-up speed regarded as mechanical disturbance was modified by the input of step and ramp types. The results of simulation were confirmed by experiments. In experiments, an individual servomotor drove the let-off system and the tension of warp yarns was measured by the backrest movement. A load-cell was connected to the spring to measure the tension of warp yarns. As a result, it was found that the ramp type in the take-up speed was better than step type in controlling the warp tension when the loom started up in both simulation and experiment.