The influence of hydrogenated olio(cyclopentadiene) (HOCP) on tensile mechanical properties of its blends with high density polyethylene (HDPE) has been studied at 20, 80, and 100 degrees C. The nominal stress-strain curves performed at 20 degrees C indicate an increase of the modulus and a conspicuous decrease of the ultimate properties increasing the HOCP content in the blend. Blends with HOCP content > 20%-wt are found to be very fragile at 20 degrees C. These behaviors have been related to the presence of the HOCP-rich phase that has the glass transition at about 55 degrees C. At 20 degrees C the HOCP oligomers act as a hardener for the HDPE. The nominal stress-strain curves performed at 80 and 100 degrees C show a strong decrease of the modulus (compared to the value at 20 degrees C), plastic deformation for all the blends tested, and rupture of HDPE and blend samples at the beginning of the cold-drawing. Moreover, the modulus and the ultimate parameters are found to decrease with the composition. The behaviors at 80 and 100 degrees C have been related to several factors : the higher mobility of HDPE molecules at these temperatures compared to that at 20 degrees C; the HOCP-rich phase that is at temperatures higher than its glass transition, and so it acts as a plasticizer for the HDPE; and the decreases of crystallinity and number of entanglements when the HOCP is added to the HDPE component.