In this article, porosity and air permeability of denim fabric produced from filament core-spun yarns with different filament fineness and yarn linear density were demonstrated. For this purpose, 110 dtex drawn textured polyester filaments with conventional, fine, and micro finenesses were used as core part, and combed cotton fiber was used as sheath part to obtain core-spun yarns with four different yarn linear density on a modified ring spinning system with the same spinning parameters. Besides the production of core-spun yarns, 100% cotton ring-spun yarns were produced as control group at the same conditions for each yarn linear density, as well. To evaluate the effect of filament fineness and yarn linear density on air permeability and total porosity, denim fabrics were obtained by using 24 yarn samples as weft at the same cover factor with four determined weft densities. Results showed that filament fineness and yarn linear density have a significant effect on total porosity and air permeability at a significance level of 0.05. In addition, high correlation (79.4%) between air permeability and total porosity of denim fabric samples was observed at a significance level of 0.01.