Abstract: Proton acceleration experiments were performed by using an ultra-short laser to irradiate gold foils of 5 m and 2.1 m in thickness. The ultra-short laser pulse had the pulse duration of 120 fs and energy of 11 mJ. The peak focal intensity remained around 61016 W/cm2. A Thomson ion spectrometer with CR39 plastic nuclear track detectors were used to observe spectra of the accelerated protons. The spectrum of the 2.1 m foil featured a single peak around 74 keV due to the inefficiency of protons. The maximum energies and efficiencies of accelerated protons increased with decreasing the target thickness. The results were explained by the energy loss of electron transmission and a geometrical effect on hot electron recirculation.