The dependence on laser intensity and pulse duration in energetic proton acceleration by irradiation of ultrashort laser pulses on a 5 μm thick copper tape target was measured. The laser intensity was varied from 8.5× 1017 W cm2 to 1.1× 1019 W cm2, and the pulse duration from 55 fs to 400 fs. The maximum proton energy increased as the pulse duration was increased while the laser intensity was kept constant. The dependence of the maximum proton energy on laser intensity and pulse duration was in good agreement with an analytical plasma-expanding model.