This study diagnoses and explores year eight students' misconceptions and mistakes in algebra. An in-depth exploration of students' misconceptions were carried out by conducting interviews based on the students' work as outlined in a cognitive map. Based on the cognitive maps, it can be seen that the students emphasize procedural knowledge rather than a comprehensive understanding of the concept. The students had misconceptions about the variable, which is the core of algebra. The students misinterpreted variables, though they sometimes succeeded in determining variable values from algebraic expressions or equations. The students made mistakes in translating literal sentences into algebraic forms, but they succeeded in completing an algebraic expression. In general, the students were able to operate algebraic forms procedurally. The exploration found that these misconceptions or errors were likely caused by their incomplete knowledge of arithmetic. This problem had an impact when they work on the transition from arithmetic to algebra concepts.