The purposes of this study were to investigate the performance of the ability of number sense for sixth graders, to summarize and analyze the patterns of errors made by school students, and explore the possible reasons for their formation, in order to understand the myths and mistakes of sixth-grade students’ learning number sense. The study results are references for teaching improving and furthering study. Both qualitative and quantitative design were employed in the study, and the number sense questionnaire designed by researcher was used to test 49 subjects who participated in the study from 2 classes of a specific elementary school in the East district of Tainan city. Based on the willingness and responses of subjects, some students were picked and then interviewed. SPSS21 and Microsoft Office Excel 2016 were used to analyze the quantitative data. By comparing the contents of the interviews, the researcher sorted the pattern of error and the cause of the error. The research results are as followings: First, the overall average performance of the sixth-grade elementary school students is low. Among them, ""the ability to understand the meaning of an operation toward a number and the operation’s influence"" is the best indication. Second, the sixth grader’s answer are divided into the following eight patterns of errors: (1) The error pattern of integer concept. (2) The error pattern of the fractional concept. (3) The error pattern of the decimals concept. (4) The error pattern of the lack of prior knowledge. (5) The patterns of stereotyped actuarial concepts. (6) Careless error pattern. (7) The error pattern of guessing. (8) The error pattern of the weak reasoning ability. Third, the reasons for the mistakes made by the sixth grade students in the school are summarized as the following five possible causes: (1) The myth concept of number sense: contains the concept of number sense and the misuse of the number sense strategy. (2) Habitual column computing learning. (3) Negative guesswork. (4) Lack of mathematical reasoning ability. (5) Lack of ability to check errors.