To bring light on various realities in accordance with the relation of mutation period and change of voice in Korean middle school girls, the author tried the series of phonetical investigation in view of the voice limit according to the age, mutation, the relation with menarche and limitation of modulation with 67 girls belonging to a chorus and ranged from 12 years and 1 month to 15 years and 10 months in age in Seoul, and the following results were obtained. 1) In the age distribution in 67 girls, 12 year-old aged group was 10, 13 year-old 37, 14 year-old 19, and 15 year-old 1. Their mean age was 13 years and 6 months. 2) In proportion to the increment of age, they showed higher frequencies in experience of mensturation and mutation. But the appearance of mutation was earlier than that of the mensturation. 3) The girls who have mensturation were 55.2% (37/67). In the age of its appearance, 9 years and 11 months was the earliest and 14 years and 7 months was the latest. 4) In view of the grade, mensturation and symptoms of mutation were developed in the second semester of the first grade in the half of the middle school girls in our country. And in the first semester of the second grade, they were during mensturation and mutation in about 70% of middle school girls. So the author considered to end the mutation period almost till the second semester of the second grade or the first semester of the third grade. 5) Considering the density of voice limit in view of the order of age and group of age, their average voice limit was 23.31 semi-tones and it's almost 2 octave. Their common voice limit was H-d¹(15 semi-tones) and representative voice limit was G-g¹(24 semi-tones). 6) Though the phenomena of descent of high pitched tone and reduction of voice limit was detected according to the increment of age, the author noticed the descent of high pitched tone about 3 semi-tone only from the group of 12 year-old age to the group of 13 year-old age. So their tendency could not be asertained. Therefore, for the mesurement of exact voice position and voice limit, the more cases and persuit which take a long time were needed in the future.