In this study, Baytube™ multi‐walled carbon nanotubes (MWCNTs) were in situ modified by graft‐polymerized 2,2′‐bithiophene. The obtained PBT‐g‐MWCNTs were used as fillers for the development of semi‐conductive nanocomposites with ethylene–octene copolymer (EOC, ENGAGE™ 8540). The methods of infrared and Raman spectroscopy, thermogravimetric analysis, and transmission electron microscopy were used to characterize the filler materials. Composite specimens with neat MWCNTs and PBT‐g‐MWCNTs at their contents of 3, 5, and 10 wt% were made by ultra‐sound assisted master batching method and their mechanical and dielectric properties were compared. The strength properties (storage modulus, tensile strength) and the elongation at break of the composites with high contents (> 3 wt%) of PBT‐g‐MWCNTs were notably improved comparing to EOC/MWCNT composites. A notable improvement by more than two orders of magnitude of EOC conductivity was determined at 10 wt% content of PBT‐g‐MWCNT compared to that of pristine MWCNTs used as the composite nanofiller.