Seam slippage is considered one of the major defects in garments, which not only reduces aesthetic appearance but also affects the strength and durability of the clothing. The factors influencing this phenomenon are divided into two categories: structural parameters (density, weave, friction, crimp, and bending stiffness) and sewing parameters (thread type, stitch density, thread tension, and needle size). Considering the importance of identifying and controlling the factors affecting seam slippage, as well as the complexity of their simultaneous interactions, the present study develops a structure based numerical model to predict the onset force of this phenomenon in plain woven polyester fabrics. In this research, 10 plain woven polyester fabric samples with different densities were prepared and sewn using lockstitch type 301 with a stitch density of 3 stitches per centimeter in both warp direction and weft direction seams, and then subjected to standard tensile testing perpendicular to the seam, with the results recorded. Finally, the numerical model was established by combining the effective components in the seam region, and its validation was performed using appropriate statistical indices. The findings indicate that increasing yarn density has the greatest effect on seam slippage force. Moreover, according to the statistical indices, the model demonstrates acceptable accuracy. The coefficient of determination (R²) for all samples was obtained as 0.579, indicating that the model can explain more than 57.9% of the variations in the experimental data. The mean absolute error and root mean square error were calculated as 13.97 N and 108.69 N, respectively. Based on the results, the proposed numerical model, relying on structural and mechanical parameters, can predict seam slippage force with acceptable accuracy. This model, while being simple and computationally efficient, can serve as an effective tool for designers and manufacturers in optimizing seam resistance and reducing costly experimental testing.