In the last years an increasing improvement in qualitative standards of human lifestyles determined a greater sense of comfort for the textiles used in indoor and outdoor activities. These textiles must fulfill the consumer requirements of comfort, fit and easy of movement. New fibres have been produced and many studies are carrying on for fabric structure optimization in order to obtain functional products. This paper presents a study made with the purpose of understanding the effect of stitch length and structure on the dimensional, thermal, air and water vapour permeability properties of knitted fabrics made of Seacell® and elastane yarns, for active sportswear applications. The results demonstrate that the thermo-physiological comfort properties such as air permeability and thermal properties can be changed by fabric construction. The loop length significantly influences these properties of the fabrics, especially air permeability and thermal conductivity. Single jersey structure is the preferred candidate for the development of the active sportswear envisaged due to better performance, particularly in terms of water vapor and air permeability.