The purpose of this study is to investigate teachers' understanding and knowledge of computational estimation, and teaching practice toward to computational estimation. There are six fifth-grade elementary teachers who participated in this study; three teachers with mathematics/ science major and three teachers with non-mathematics/science major. Each teacher received the interviews for three times and the interview time was about 90 minutes. The major findings are as follows: In the teacher's knowledge about computational estimation aspect, six teachers could explain the meaning of computational estimation, and use computational estimation strategy to solve problems flexibly. Their computational estimation strategies to solve problems include front-end, rounding, compatible number, special number, use of fractions, nice number and distributive property. All of the teachers use special numbers; five of them use rounding and compatible number strategies. Four teachers use nice numbers. Only one teacher uses front-end strategy and distributive property. The results indicate that the teachers with a mathematics/science major were more abundant of the knowledge about computational estimation relatively. The teachers who are non-mathematics/science majors but were extremely interested in mathematics had initial understanding toward the related knowledge of computational estimation. All these teachers incorporated mathematics course with computational estimation, or integrated with other mathematics concepts, required students to examine the answer and its rationality by computational estimation, introduce the life situation with teaching to improve student's learning motivation. However, the teachers who are non-mathematics/science majors and not interested in mathematics have less computational estimation strategy for their teaching.