In this research, the effects of fabric constructions having surface extension ability and different fibre construction on model and pattern designs were examined in women's trousers in ready wear sector. The aim of this research is to determine the effects of especially cotton fabrics having different extension ability and different fibre construction on trousers patterns' model and pattern design process. In the scope of the research, the prepared trousers patterns were applied on 6 different fabrics selected From between fabric being denim, cotton, having washing and elasticity and they were produced in the same conditions in file establishment medium. Later they were treated with washing operation and evaluated on 24 live mannequins through evaluation scale. Trousers which showed utmost change within 6 different fabrics were re-produced with the amended pattern and evaluated. In certain parts (hips, knee, waist etc.) of denim surfaces used in the research, it was determined that the applied pattern took form proportional to elasticity of the fabric and couldn't keep form in certain parts (waist, loins etc.). It was observed that the ratio applied on the pattern did not become adequate according to elastane ratio of denim surfaces, whose elastane ratios are different, selected in the research. in this research, it has been put forward to necessity to carry out necessary amendments on the patterns according to the elastan ratios and fabric constructions in order to design suitable patterns to the bodies.