At present, many orthodontists regard the root parallelism of the posterior teeth important not only in diagnosis and treatment planning but also for predicting posttreatment stability. To evaluate it, they usually refer to the orthopantomogram. At this study, 97 samples were collected from students of Yonsei University, who have well-proportioned face, Angles´s class I canine & molar relationships and no crown axis deviation of the posterior teeth including canines. Reliability of the root parallelism observed from orthopantomogram was evaluated by comparison with 45° oblique cephalogram. The results were as follows : 1. In comparing the differences between anglular measurements in 45° oblique cephalogram & orthopantomogram with 5°, those to mandibular plane were significantly less than to occlusal plane in number of items which showed less differences than 5°. 2. Compared the root parallelisms in the orthopantomogram with those in 45° oblique cephalogram with 1% significance level, parallelism between upper canine & 1st premolar, lower canine & Ist premolar, lower 1st premolar & 2nd premolar, and lower right 2nd molar & 3rd molar showed statistically significant differences. 3. When the significance between the differences of the root parallelism between above two kinds of film and 5° was verified by two sided paired t-test, more or less large difference was shown between lower right 2nd molar & 3rd molar, a little larger than 5° between lower canine & 1st premolar, smaller at the rest of them. 4. In 45° oblique cephalogram, lower canine & 1st premolar showed convergent root arrangements each other, while in orthopantomogram they were divergent each other. All the others except them showed convergency on the upper, divergency on the lower in both films.