Purpose: This study measured vowel formants and vowel space area from 3 to 6 year-old Korean children in Jeju Island to investigate their characteristics of vowel production and speech intelligibility. Methods: A total of 80 children (10 boys and 10 girls for each age) participated. For obtaining the formants, 7 Korean vowels (/a/, /e/, /i/, /o/, /u/, / m/, /^/) were used. Vowel space areas (triangular, quadrangular) and speech intelligibility were calculated and compared based on gender and age. Results: Results showed that F1 and F2 of / e/ and /i/ vowels were significantly different as age. In particular, front vowel /e/ and /i/ was significantly lower in 3 years of age but was gradually increased with age. For gender, F2 of vowel /a/ and /e/ were significantly higher in girls while F2 of vowel /i/ was significantly higher in boys (p < 0.05). With respect to vowel space area, significant differences were found in ages (p < 0.05) whereas no differences in gender (p > 0.05). Speech intelligibility was significantly different in age (p < 0.05) while no significant difference was observed in gender (p > 0.05). Significant relationships were found between triangular vowel space areas (r = 0.299, p < 0.01) or quadrilateral vowel space areas (r = 0.570, p < 0.01) and speech intelligibility. Conclusion: Accordingly, quadrilateral vowel space area composed of / i-e-a-u/ was more sensitive than triangular vowel space area composed of /i–a-u/, reflecting the vowel movement space and speech intelligibility objectively.