The effects of poly-3-octylthiophene (P3OT) functionalized multiwalled carbon nanotubes (MWCNTs) on the structure as well as stress-strain characteristics and electrical conductivity of ethylene-octene copolymer with 17% of octene comonomer content (EOC17) are investigated. According to Raman spectroscopy and TEM analysis, thiophene groups have been successfully grafted onto the surface of MWCNTs. EOC17 nanocomposites with P3OT functionalized MWCNTs show simultaneously high values of stress at break and strain at break, even at high nanofiller content, being respectively 2.5 and 3.5 times higher than the values determined for the systems containing pristine MWCNTs. It is also demonstrated that at functionalized filler content of 10 wt%, twofold increment of the modulus of elasticity is observed. In addition to that, P3OT functionalized MWCNTs containing EOC nanocomposites possess up to 279 times greater electrical conductivity in comparison to the systems with pristine MWCNTs.