Research shows that children’s mathematics anxiety negatively impacts their performance on mathematics tests (e.g., Chernoff & Stone, 2012). However, no research to my knowledge has investigated how children’s mathematics anxiety impacts their conceptual understanding of arithmetic. The current study investigated the characteristics and development of Grades 4, 5, and 6 children’s mathematics anxiety and how it impacts their conceptual understanding and application of arithmetic using arithmetic concepts. For comparison, the current study also investigated how children’s mathematics anxiety impacts their arithmetic fluency using timed mathematics tests. As an exploratory and secondary component, the current study examined teachers’ mathematics anxiety and how it impacts their students’ mathematics anxiety, conceptual understanding of arithmetic, and arithmetic fluency. Results showed that children with higher mathematics anxiety had weaker conceptual understanding of arithmetic and weaker arithmetic fluency compared to children with lower mathematics anxiety. In particular, children’s mathematics anxiety had a greater impact on their arithmetic fluency compared to their conceptual understanding of arithmetic. It also showed that females had higher mathematics anxiety compared to males. However, there was no significant effect of grade on children’s mathematics anxiety. The exploratory component showed that teachers’ mathematics anxiety did not impact their students’ mathematics anxiety, conceptual understanding of arithmetic, or arithmetic fluency compared to teachers with lower mathematics anxiety. Grade 4 teachers had higher mathematics anxiety compared to Grade 5 teachers. However, there was no significant difference between Grades 5 and 6 teachers’ mathematics anxiety. Female teachers also had higher mathematics anxiety compared to male teachers. Lastly, students were unaware of their teachers’ level of mathematics anxiety. To help improve children’s performance on mathematics tests, future research should focus on early identification and corresponding interventions for children and teachers with mathematics anxiety.