We diagnosed the polarization characteristics of Terahertz (THz) emission from a femtosecond laser filament in air in the
forward direction by using a wire grid polarizer and E-O sampling technique. The experimental results have demonstrated that the investigated THz pulse is elliptical polarized. It indicates that there is a new generation mechanism for THz emission from a laser filament other than the transition-Cherenkov radiation which is radially polarized. The generation mechanism could be through four-wave optical rectification or second-order optical rectification inside the filament due to the spatial asymmetry of the generated plasma column in the experiment.