The purpose of this study was to investigate the solving performance of division for third graders. By generalizing their error patterns and analyzing the possible reasons for making mistakes in order to know students’ difficult part in learning. The result could be reference for teachers. The researcher designed a specific test of “Division” including 19 questions in 12 types. The participants selected to complete the test were 46 third graders from a public elementary school in Rende District, Tainan City. After testing, some of them were interviewed according to their performances and willingness. The statistic tools of SPSS 21.0 were used to analyze the quantitative data linking the details of interview to figure out the error patterns and the possible reasons for their mistakes. The researcher analyzed the data and then obtained the following conclusions: The main error reasons of division operation for third graders: a. Misunderstanding the relation between dividend and divisor, and lacking of the necessity of sharing completely in division. b. In the column form of division, students easily had errors in calculation of multiplication and subtraction. c. Students’error in estimating quotient resulted from their unfamiliar concepts of place value and decomposition. d. Students were confused by different types of questions of complementing zero in quotient. e. Students were incapable of understanding the questions. f. Students’ incomplete arithmetic skills caused errors. g. Students misused irrelevant concepts for solution. h. Students’irrelevant omission caused errors.