Copper oxide (CuO) nanorod-like structures made of spherical nanocrystals were synthesized at moderate temperature (80°C) starting from CuCl2·2H2O crystals in a water/n-heptane microemulsion stabilised by the nonionic Brij-30 surfactant. Whole Powder Pattern Modelling of the X-ray diffraction pattern shows absence of linear and planar defects with crystalline domains in the range of 4–8 nm. A linear correlation between the average size of the particles and the quantity of water in the system was observed: all synthesised specimens show a large blue shift of the energy bandgap (up to 2.7 eV versus 1.2 eV of bulk CuO) resulting from quantum confinement effects. The mechanism of growth of the spherical nanoparticles into nanorod-like structures has been elucidated.