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Junior high school students in Indonesia are still experiencing misconception number sense. Therefore this  explorative study aims at  identifying  misconceptions on  number  sense experienced  by junior  high school students and  factors causing  misconceptions.  This  study used 3 seventh grade students as subject. The instruments used in  this study are two-tier test and an unstructured interview. The subjects were asked to do the test and then their works were analyzed qualitatively. To  validate  the analysis  undertaken, an  unstructured interview was taken place toward each subject based on the work that had been done. Results showed that Subject  had misconceptions in including place value of decimal, comparing / ordering fractions, changing mixed fraction into decimal, sum, reduction, multiplication, and division of fractions. Factors causing  misconceptions is the junior high school students in Indonesia in solving the problem of fractions still treats fractions and decimals such as integers, students do not understand how to change the fraction, students are confused to perform fractional operations, students are not trained how to use intuition and also students do not understand the concept of fractions. If the misconception of this number sense is allowed to continue the students will have difficulty in learning the next mathematics material. For that reason the curriculum in Indonesia should give opportunity to students and teachers to be able to learn about the number sense. And also teachers must be creative and able to use variations of concepts in teaching.

misconception, number sense, two-tier test, fraction