Due to the environmental issue, natural fibers are day by day becoming attractive to researchers. Natural fiber contains cellulose, hemicelluloses, lignin etc, which are hygroscopic in nature and biodegradable. The lack of surface feature diminishes its properties. So, the surface properties of the jute yarns need to be modified. In the present study, jute yarns were cleaned using 2% detergent and chemically modified by 5, 15 and 25% NaOH solution both at room temperature and 700C for 2 hours and dried in air. The structural and morphological studies of the treated and untreated yarns were carried out using Fourier transform infrared spectroscopy (FT-IR) and Scanning electron microscopy (SEM). The thermal and mechanical behaviour of the yarns were analyzed using Differential scanning calorimetry (DSC) and Instron Universal testing machine. The results show the improvement in mechanical strength of the yarns due to the change in crystalinity after alkali treatment. Also, the thermal decomposition temperature of raw jute yarns decreased from 357.30C to 349.60 C after alkali treatment.