This is a research study that analyzes, through a questionnaire survey, special school teachers’ practical knowledge in the number area of mathematics regarding students with intellectual disabilities. A sample of 237 special school teachers from Incheon, Gyeonggi, Chungcheong or Gangwon Province were sent a 5-point Likert scale questionnaire comprising 69 questions about their practical knowledge in the number area of mathematics. The t/F tests, one way ANOVA, correlation analysis, and multiple regression analysis were utilized in this study. The findings summarize the following results. First, special school teachers’ practical knowledge contents in the number area of mathematics with regard to students with intellectual disabilities was influenced by background variables such as gender, special education career, and level of education. Second, the sub factors of special school teachers’ practical knowledge contents in the number area of mathematics concerning students with intellectual disabilities exhibit a high degree of static correlation. In addition, educational goals and value knowledge are significantly associated with knowledge of the learner, knowledge of the teaching and learning environment and curricular knowledge. However, knowledge of subject matter and knowledge of teaching-learning did not show a statistically significant association with each other. Based on these results, discussions and conclusions were drawn.