In this study, we investigated the value of a concept map marking guide as an alternative formative assessment tool for science teachers to adopt for the topic of energy. Eight high school science teachers marked students' concept maps using an itemized holistic marking guide. Their marking was compared with the researchers' marking and the scores of a multiple-choice diagnostic test. Statistical analysis revealed that teachers evaluated students' concept maps consistently, but the concept map scores were only mildly correlated with the multiple-choice test scores. We explored what each assessment method revealed in terms of students' understanding with one student's performance as a showcase. Teachers' views on using the marking guide of concept maps as an assessment tool were also included. We discuss the value of the concept map marking guide as a formative assessment tool for science teachers to create and modify in order to encourage students' conceptual learning.