In this article, the effect of carbon nanotube’s (CNT) surface modification on mechanical, thermal, and morphological properties of nanocomposite with different content of CNTs was studied. CNT/high-density polyethylene (HDPE) nanocomposites were prepared by a mini twin-screw extruder. To modify the dispersion of MWCNTs in HDPE, the surface of CNTs was functionalized by HNO3 and stearic acid (SA). The tensile tests results show that addition of 5% of acid-treated CNT in HDPE cause to increase in Young’s modulus and yield strength up to 4% and 6%, respectively; as compared to unmodified CNT/HDPE nanocomposite. The presence of the SA on CNTs can lead to slight decrease of the CNT-SA/HDPE strength properties; when compared to pure CNT/PE nanocomposite. However, there is a fracture toughness enhancement in CNT-SA/HDPE up to 10% compare to unmodified CNT/PE nanocomposite. Furthermore, the calculation of the stress whitening zone of all samples verifies the fracture toughness data of CNT/HDPE nanocomposites. Scanning electron microscopic images showed that the best dispersion of CNTs in the HDPE matrix was seen in CNT-SA/HDPE nanocomposite.