The interaction of ultra-short high power lasers with gas targets is an important way to produce high power terahertz (THz) sources. Based on the model of ionization currents, the THz radiation generation is considered by use of two color lasers at 0.8 mm wavelength and its half-harmonic 1.6 mm co-propagating in a gas target. The influences of the types of gases, the laser amplitudes and the relative phase difference between the two laser pulses on the ionization currents are discussed. Compared with the case with 0.8 mm laser and its second harmonic, the THz radiation amplitude is enhanced twice under the same incident laser amplitude. The effect of the angle between the two laser polarizations is also studied. When the two polarizations are perpendicular to each other, the ionization current direction is along the polarization direction of the half-harmonic laser, even though its amplitude is much lower than that of the fundamental laser.