Formant frequencies of a vowel are generally extracted from midpoints or central sections on the time axis. Nasal vowels present a challenge for obtaining stable formant frequencies, as the midpoint often falls in an anti-formant section where vocal energy is lost through the nasal cavity. This study proposes a stable section for extracting nasal vowel formant frequencies using difference thresholds, which identify a vowel as being distinct when F1 is above 60 Hz and/or F2 is above 200 Hz. For the experiment, 481 disyllabic words (232 nasal vowels and 294 oral counterparts) are selected from an online French-Korean Dictionary. Each vowel is divided into 10 intervals, and the stable section is identified as one or more continuous intervals with lower frequencies than the difference thresholds. The results show that the stable section for nasal vowels is identified in 20%~50% of the vowels, while the stable section for oral vowels is identified in 20%~80% of the vowels.