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In recent years metacognition was discussed as significant concept in mathematics education. This research aims to investigate student’s metacognition and its activity in solving open-ended problem on geometry. Qualitative research with case study strategy was used in this research. Subjects in this research student of 11^th grade chosen with purposive sampling based on mathematics achievement and gender. Data in this research was collected through interview based on geometry task. The results show that at each stage of problem solving, students with high mathematics achievement both male and female have a very complete metacognition execution. But the level of completeness of metacognition activity of male students is higher. On the other hand the implementation of metacognition of students with low mathematics achievement both male and female students is at a low level, the variation in the implementation of metacognition activities is much lower than students with high mathematical skills. But in female and male students with low mathematics skills have the same variations of metacognition activity.