Abstract
Geometry is one of mathematics’ material that explain about object in dimension one, two, or three. Geometry can give student to visual’s ability of their objects. Student that have good visual’s ability will effect imagination’s ability too, and it’s reverse. Geometry is mathematics’ material that is difficult to understand for student and to teach for teacher. This qualitative research is aim to (1) know students’ Mathematical mistakes to solve in geometry problem, and (2) know causes students’ mistakes in solving of geometry problem. The result show that (1) mistakes of geometry conceptual and procedural by middle and high student, (2) mistakes of procedural is more than conceptual, (3) mistakes conceptual is determine formula of area in dimension two, (4) mistakes of procedural is precisely draw plane, and count its area such as square, rectangle, trapezoid, and (5) causes students’ mistakes are student can’t read ruler and student is not understand concept of concept, and learning is not meaning-full. Keywords: mathematical mistakes, geometry, mistakes procedural and conceptual