The first-principles calculation method has been used to obtain structural and electronic properties of few-layer graphenes (FLG's) with and without a cross-film external electric potential, Vext. It is found that the AB stacking is more favorable than the AA stacking and the calculated layer spacing for the two-layer AB stacked FLG is only 2.725 Å, which is substantially reduced from that of the bulk graphite of 3.257 Å. The two-layer AB stacked FLG is found to exhibit a semi-metal-semiconductor transition under Vext qualitatively in agreement with previous studies. However, the energy gap, Eg, is not limited at 0.3 eV as obtained in previous first-principles calculation due to the reduced interlayer spacing, The threshold of the semi-metal-semiconductor transition is 0.04 Volts. Vext also induces Eg's in 3- and 4-layer AB stacked FLG's. However, in these FLG's the induced Eg's are small within 0.1 eV.