The effect of a laser prepulse on the generation of proton beams is compared between plastic and metal targets by irradiating a 30fs, 2.4×1018W∕cm2 Ti:sapphire laser pulse. Proton energies generated from both target materials increase as the pulse duration of the laser prepulse decreases. However, it was found that there are distinct differences with respect to target materials. In the case of aluminum targets, as target thickness decreases, proton energy gets higher, which is well described by an isothermal expansion model. However, in the case of Mylar® targets, no such dependence on target thickness could be observed, and the highest maximum proton energies are higher by factors of 1.5 to 3 than those from aluminum targets or those predicted by the isothermal expansion model. Such characteristics of the proton beams from Mylar® targets can be accounted for by a bulk acceleration model, or acceleration by a resistively induced electric field.