Seam quality is very important aspect of any form of textile assembly using seam. Seam quality is characterized by seam strength, seam elongation, seam efficiency and seam puckering. The purpose and objective of this study was to investigate and scrutinize the impact of commercial sewing thread counts and stitch densities on seam strength, seam elongation and seam efficiency on medium-heavy and heavy weight cotton denim fabrics. For the experimental work, core spun polyester sewing thread in three different counts (Tex-60, Tex-90, and Tex-150) was selected. Superimposed seam using Stitch Class-300 (Lock Stitch) at two different stitch densities (SPI 10 and SPI 13) were used on 3/1 right handed warp face twill woven cotton fabric. The effect of different sewing thread size and different levels of stitch densities was assessed on the selected seam parameters. The interaction effect of the independent variables was also investigated. The experimental results were evaluated statistically using variance analysis (ANOVA) and regression models which correlate seam quality with stitch density and sewing thread size for both the type of fabrics. The findings of the study revealed that for all the three seam parameters, an increasing trend was seen with the increase in the sewing thread count at higher value of stitch density on both the fabrics. Statistically, it was found that for heavy weight denim fabric, the independent variables do not have any significant effect on the seam quality whereas for medium-heavy weight denim fabric some of the independent variables have significant effect on the seam quality. It was seen that statistically there was significant interaction between stitch density, sewing thread count and fabric weight on strength, elongation and efficiency of superimposed seam.