This paper analyzes students' errors at X MIPA 4 SMAN 1 Ampek Angkek, and it causes in solving vector based on Castolan. The type of research is descriptive with a qualitative approach—data analysis through reduction, display, and conclusion. The result showed that 1) around 66,66% of students made a conceptual error in vector operation, 33,33% in measuring the angle between two vectors, also 55,55% in orthogonal vector. 2) around 44,44% of students made a procedural error in measuring the angle between two vectors. 3) around 11,10% of students made a technical error identifying three collinear points. The cause is students' carelessness and does not recheck the answers. [ABSTRACT FROM AUTHOR]