We study the nanoelectromechanics in a quantum wire connecting two reservoirs irradiated under an external terahertz (THz) electromagnetic field. Using the free-electron model at ballistic regime we calculated the two-level electron wavefunction and the Rabi oscillation of electron probability between the two levels. Using a statistical operator we calculated the nanoelectronic force in the wire. The result consists of two terms, which corresponds to the case with or without external field. The term without field parameter is consistent with the existing experimental observation and theoretical calculation, and the other term from the external field can be explained by the quantum coherent theory.