In this study, we present three experiments carried out at the EISCAT (European Incoherent Scatter Scientific Association) heating facility on October 29 and 30, 2015. The results from the first experiment showed overshoot during the O-mode heating period. The second experiment, which used cold-start X-mode heating, showed the generation of parametric decay instability, whereas overshoot was not observed. The third experiment used power-stepped X-mode heating with noticeable O-mode wave leakage. Parametric decay instability and oscillating two-stream instability were generated at the O-mode reflection height without the overshoot effect, which implies suppression of the thermal parametric instability with X-mode heating. We propose that the electron temperature increased because X-mode heating below the upper hybrid height decreased the growth rate of the thermal parametric instability.