The precipitation behavior in an Al-Cu alloy isothermally annealed at 373K～493K was studied, using the Vickers microhardness tests, Differential scanning calorimetry(DSC) and Transmission electron microscope(TEM) observations. An additional endothermic peak was found in DSC measurements which may be attributed to the θ″-phase independent from the formations of the G.P.(Ⅱ) and the θ′-phase at the aging temperatures. High resolution transmission electron microscopy (HRTEM) revealed that the G.P.(Ⅰ) is formed at a very early stage of isothermal aging at 463K. Comparing the Vickers microhardness and TEM images, it was concluded that G.P.(Ⅱ) is mainly responsible for the peak hardness.