Under external transverse electronic fields, the electronic and magnetic properties of silicon carbon nanoribbons with zigzag edge (ZSiCNRs) and armchair edge (ASiCNRs) are studied by using the first-principles projector augmented wave (PAW) potential within the density-function theory (DFT) framework. Our results for the narrow ZSiCNRs by the VASP computer code are in good agreement with those of Lou and coworkers. Thus, in this paper we showed the new results for the wide ZSiCNRs as a complementary study as well as for the narrow and wide ASiCNRs. The ground state of the ZSiCNR is a ferrimagnetic semiconductor with two different direct bandgaps for the spin-up and the spin-down channels near the zone boundary Z, and converts to a ferromagnetic metal when a suitable transverse electric field is applied. However, the ground state of the ASiCNR is a nonmagnetic semiconductor and its bandgap decreases with increasing absolute value of the field strength. After a suitable critical field, the ASiCNR changes to a metal, and the larger ribbon width, the larger decrease rate and thus the smaller critical field.