Many methods have been used for investigation of stress relaxation characteristics of yarn and fabric. Theoretical, Statistical and Rheological models are such these methods. Mechanical models play very important role in investigation of yarn and fabric behavior. Viscoelastic models including spring (Hook Low) and dashpot (Newton Low) are mostly used for investigation of mechanical behavior of textile material. Multi combination of spring and dashpot make different viscoelastic models which ones could consider viscoelastic behavior of material such as stress relaxation. The aim of this project is: Theoretical and empirical investigation of stress relaxation of yarn and fabric with multiloball cross section. For this purpose three yarns sample with the same count but different internal structure have been selected. The first sample has round cross section, 2nd sample is tetra channel (tetra lobal) cross section and 3rd one is hexa channel (hexa lobal) cross section. All samples were then texturized in the same condition and knitted in circular knitting machine in order to produce fabric. Tensile strength and stress relaxation of yarns and fabrics have been tested by Zwick apparatus, and the results compared and analyzed with each other. The effect of cross section shape was also considered on stress relaxation characteristics of yarn and fabrics. Theoretical and empirical results were compared. The standard linear model showed the best fit for the relaxation behavior of yarn and fabrics. The results also showed that increasing of channel numbers leads to decrease of the strength and elongation of the yarn. In the case of yarn an acceptable correlation was observed between theoretical and experimental values of the model parameters. However, in the case fabrics no significant correlation was observed between the theoretical and experimental values of model parameters.