A tunable dual-wavelength external cavity laser diode (2-lambda-ECL) was used to generate tunable coherent cw-terahertz (THz) radiation by photomixing in a biased photoconductive antenna fabricated on a low-temperature-grown (LTG)-GaAs film. The 2-lambda-ECL oscillated at the two output modes simultaneously with a frequency separation from 250 GHz to 2.1 THz in the same cavity. We demonstrated that the frequency of the emitted THz radiation corresponded to the difference frequency between the two modes of the diode laser. The tunable 2-lambda-ECL, due to its compact design and ease of optical alignment, is a promising laser source for the generation of tunable cw-THz radiation by photomixing.