This study investigated Mathematical Knowledge for Teaching (MKT) in differentiation at the high school level. According to the revision of the 7th curriculum, high school students in the social and human curriculum have to study differentiation; hence, there is a reason to investigate the analytic frame of measuring Mathematical Knowledge for Teaching (MKT) in differentiation. Through a survey of precedent studies of student understanding of differentiation, teachers\' knowledge was investigated and analyzed. The questions of the survey were created by considering both the teachers\' situations and the questions in precedent studies. To investigate the teachers\' MKT of differentiation, 36 high school teachers were surveyed. The analysis results are as the follows: since the mean score of the total MKT is 18.61 and the standard deviation is 3.192, teachers have good MKT of differentiation. In general, high school teachers have a high degree of MKT although they don\'t teach differentiation. There is a need, however, to focus on the significant gap between the highest score (23 points) and the lowest score (11 points); 72.2% of teachers received high scores but two teachers achieved only 11 points, one teacher only 12 points, and one teacher 13 points. These four teachers, who recorded 13 or lower points, need to be retrained. The results from the subdomains of MKT are as the follows: first, the highest SMK was 16 (4 teachers achieved this score) and the lowest SMK was 6; the mean score is 12.97. Eleven teachers scored 15 points, and six teachers received 13 points. One teacher received a very low score, with two teachers scoring a little better at 9 points. Since the number of teachers who achieved a full score was four (11.1%), we conclude that, in general, teachers have a high level of SMK. Second, the highest KCS was 4 and the lowest KCS was 0; the mean was 2.94. Although ten teachers (27.8%) achieved the highest score, there was one teacher who received 0 points. Third, the highest KCT was 5 (12 teachers achieved this score) and the lowest KCT was 2; the mean was 4.14. Eighteen teachers achieved 4 points, five teachers 3 points, and one teacher 2 points. Significantly, the teacher who received the lowest score in KCT, received the lowest score in SMK, and also in total MKT. This teacher needs to be reeducated.