To study the calcification of second molars in skeletal class II malocclusion, the author examined cephalograms., orthopantomograms, periapical films of 391 children from 7y 0m to.15y 11 m years old who had skeletal class II malocclusion, and observed the difference in the calcification stage between upper and lower second molars. . The results areas follows. 1. The mean ages of crown completion of upper and lower second molars are 8.7 ± 1.75, 8.8 ± 1.13 in. boys, and 8.4 ± 0.81, 8.5 ± 0.91 in girls. 2. The mean ages of root completion of upper and lower second molars are 14.0 1.09, 14.5 ± 0.52 in boys, and 13.7 ± 1.15, 13.8 ± 1.18 in girls. 3. The calcification stages of upper second molars are more advanced than those of lower second molars at 5% level in both sexes.