Eri silk fibers are produced by the silkworm Samia cynthia ricini, which has been extensively cultivated in several tropical countries due to its adaptability to elevated temperatures and humidity, as well as the ample availability of its food sources. Currently, we have developed six new strains of S. ricini with extraordinary cocoon productivity and the ability to survive in marginal/extreme areas (high temperature and low humidity), namely Joglo, Jopati, Pasopati, Prasojo, Progo, and Tawang strains, respectively. This research analyzes the physical characterization of these Eri silk fibers, including the thermal characteristics, crystallinity structure, morphology, and functional groups of the silk fibers. The surface morphology of the degummed Eri silk fibers reveals the presence of separated fiber filaments with a smooth surface, ranging in diameter from 14 – 33 µm. The tensile strength and elongation of the silk fibers from the new strains do not show a significant difference (P > 0.05 %), around 350 – 497 MPa and 15 – 26 %, respectively. In addition, the Fourier transform infrared spectrometer was also utilized to investigate the structural differences of Eri silk fibers. The results show absorption bands at 1623 cm-1, 1513 cm-1, 1219 cm-1, and 703 cm-1 indicating vibration of C=O (Amide I), N–H (Amide II), and C–N (Amide III) groups indicate the presence of β-sheet structures in the silk fibers. X-ray diffractograms of Eri silk fibers show peaks at 16.54° and 20.40° that correspond to the β-sheet crystalline structure. The new strain fibers have no significant structural disparities compared to the general S. ricini silk fibers. The thermal characteristics of the Eri silk fibers from the new strains are similar, with water removal occurring at temperatures around 100 to 125 °C and degradation temperatures around 369 – 373 °C. However, the Pasopati strain exhibits the highest heat capacity increment (ΔCp), 1.266 J g-1 °C-1. Based on the comparative results, Eri silk fiber from novel strains of S. ricini reported can be utilized more extensively in the textile industry and biomaterial applications.