The purpose of the study was to determine the students' error in learning quadratic equation. The samples were 30 form three students from a secondary school in Jambi, Indonesia. Diagnostic test was used as the instrument of this study that included three components: factorization, completing the square and quadratic formula. Diagnostic interview was also used to identify at which level students' errors occur in solving problems. The type of error is based on Newman Error Hierarchy Model that includes reading type error, comprehension, transformation, process skill, and encoding error. Data was analyzed using descriptive statistics: percentage and frequency. The findings showed that most students make error in transformation and process skill in solving quadratic equations. There was no error found in reading. The number of students who made encoding error and carelessness was small. The students' error in solving quadratic equation was due to their weaknesses in mastering topics such as algebra, fractions, negative numbers and algebraic expansions.