A primary amine, salicylaldehyde and copper salt were combined to prepare the bis(N-benzyl-salicydenaminato)copper (II) complex. The copper (II) complex was then used as a precursor to synthesize mixed phase morphological copper oxide nanoparticles via thermal decomposition method using trioctylphosphine oxide as a capping molecule at different temperatures of 120, 180, and 240 oC. The XRD patterns of copper oxide nanoparticles synthesized at lower temperatures exhibit a mixture of monoclinic structure of CuO whereas the nanoparticles synthesized at higher temperature reveals the peaks that are attributed to mainly face-centered-cubic metallic Cu. The TEM images showed spherical particles that were increasing in sizes when the temperature was raised.