The initial stages of the oxide formation were investigated by oxidizing Cu(100) thin films in a specialized in situ ultra-high vacuum (UHV) TEM at oxygen partial pressure of 5x 10(-4) torr and a temperature range of 150degreesC to 1000degreesC. Epitaxial Cu2O islands formed on Cu(100) where the morphology can be either triangular, huts, rods or pyramids depending only on the oxidation temperature. Of particular focus was the formation of nanorods at 600degreesC, that was observed to change from initially square shaped islands to elongated islands at a critical size of 114nm. The experimental data on the elongation of Cu2O islands agree with the energetic calculations based on the balance between surface/interface energies and the elastic strain relief in the three dimensional islands.