Traditional ferrography only emphasizes on the characteristics of individual big debris. For diagnosing and monitoring the condition of machines, the traditional ferrography encounters trouble. Ferrography based on debris group theory thinks that there is a one-to-one correspondence between group characteristic of debris group and the operating condition. For this reason, the real wear condition will deduced by studying the group characteristic of debris group in the ferrogram. By drawing out the criteria of the studied machine, the failure of the machine components will be predicted. Theoretical analysis and experimental research show that the operating condition predicted by debris group theory are consistent with the practical condition.