Generation of ELF and VLF waves via a thermal instability process in a HF-modulated polar electrojet has been investigated. It is shown that a positive feedback through the electron-neutral collisional heating process can cause the transient response of the plasma to the modulated HF heater to grow exponentially. The threshold fields of the instability under normal electrojet conditions are found to be about 2.25 V/m and 1.13 V/m for the operation of the o-mode and x-mode heaters with a 50% duty cycle, respectively. For a heater wave field of, e.g., 1.5 V/m, the instability can be excited by the x-mode heater within a few tens of millisecond. The predicted dependence of the ELF/VLF radiation amplitude on the percentage of the duty cycle of the HF modulation is identical to that of the recent observational results of Barr and Stubbe [1991b>. ¿ American Geophysical Union 1993