A thermal oscillating two-stream instability as the generation mechanism of electron Bernstein/upper hybrid waves by the o-mode HF heater wave in the high-latitude ionosphere is investigated. The purely growing decay mode is described by the fluid equations, while the kinetic equations are employed to derive the coupled mode equations of the electron Bernstein/upper hybrid sidebands. It is found that the instability can be excited in the altitude regions both above and below the upper hybrid resonance layer. The parametric excitation of electron Bernstein waves is in general not effective. On the other hand, the upper hybrid sidebands of the instability excited in the ''below'' region have been considered by Leyser (1991) to be the pumps for the parametric excitation of downshifted maximum (DM) lines of the stimulated electromagnetic emissions (SEEs) observed in the ionospheric heating experiments. The detail analysis of the proposed work shows that the instability zone of upper hybrid waves below the upper hybrid resonance layer becomes very small when the heater frequency f0 is operated near three times of the local electron cyclotron frequency fc. Thus the source wave of the DM lines is effectively suppressed. This result explains the phenomenon of quenching of the DM feature in the SEE spectrum as f0 is adjusted near 3fc in the ionospheric modification experiments. ¿ American Geophysical Union 1994