Herein, we fabricated bead-free isotactic poly(4-methyl-1-pentene) (PMP) nanofiber membranes and characterized their thermo-mechanical properties. PMP nanofiber membranes were electrospun and heat-treated at 180 and 220 A degrees C, and thermally treated under load. The report investigates the effect of thermal treatments on the morphology, degree of crystallinity and mechanical properties, improving the mechanical properties of PMP nanofibers. Prepared nanofibers were investigated by SEM, DSC, XRD and mechanical properties. The mechanical properties demonstrate a tensile strength, an elongation (%) and a Young's modulus of the nanofiber membranes. The DSC and WAXD analysis shows an increase of degree of crystallinity with thermal treatment. Thermally treated nanofibers under load demonstrate 4.1 times higher tensile strength and 14.1 times higher Young's modulus than PMP fibrous membrane. Thermally treated nanofibers under load at 200 A degrees C did not retain their structure and fuse with neighboring fibers, because it almost reached the melting temperature of (230 A degrees C).