This study aims to evaluate cognitive conflict in constructing mathematical concepts, based on thinking errors. The data were collected through observations of words or sentences, leading to the derivation of qualitative outputs. Furthermore, the results showed that the two selected subjects experienced cognitive conflicts regarding thinking errors in mathematical concept construction. This selection process was conducted through the provision of the questions obtained in the first semester of mathematics, at Universitas Negeri Makassar, Indonesia. These questions consisted of several indicators, with each of them having three components, namely the main, tracking, and supporting items. Based on the results, two subjects experienced thinking errors in constructing the concept of algebraic root form addition. This emphasized the errors in placing concepts (misplaced), pseudo-thinking, and misanalogy. A conflict was also observed between the initial concept knowledge of using variables in algebraic addition and the new principles understood through the root summation in algebra. Furthermore, there was also a conflict between the concept understanding he had in terms of the addition operation of the root form and a new understanding of the addition operation of the root form in algebra.