This study investigates the criteria undergraduates adopt to justify their claims proposed after laboratory experiments. There are two categories of justifications in the current literature, empirical consistency and plausibility of claims , but observations of college juniors in the laboratory demonstrated the need for a third category, observation reliability . This assumption was further tested by analyzing the warrants undergraduates wrote to justify their claims formed after laboratory experiments. Three students' justification criteria were identified, that is, empirical consistency , plausibility of claims , and observation reliability . The most frequently used criterion is plausibility of claims to justify good results , while that is observation reliability to justify bad results . Moreover, multiple justification, which means more than one attempt being made to justify a given claim, was also found. It reveals that multiple justification, rather than single justification, is suitable for students to make scientifically acceptable claims. The implications and suggestions of this study are also discussed.