This researchaimed to describe the student’s mathematical critical thinking skill viewed from
their activeness in SMP N 1 Sumbang. The subjects of this research were the eight graders of
class C. There were two respondents for each activeness type in organization who were taken
using purposive sampling. The date were collected using questionnaire, test, interview, and
documentation to be then analyzed using Miles and Huberman model that included data
reduction , data display, and conclusion, whiloe triangulation test was used as the data validation
test. The result showed that student who actively join organization was able to master three
indicators of critical thinking ; student who did not actively join organization wa able to master
three indicators of critical thinking ; whilw student who did not join any organization was able to
master four out of five indicators.