This study attempts to provide new insights into the historical relationship between -bu- and -mbu-. Based on the examination of the headwords in Yuzhi Qingwenjian 御製淸文鑑, I analyzed the meanings of the verbs according to the types of correspondences of the active verbs and their passive or causative counterparts. As a result, the suffix -bu- is confirmed as the representative passive or causative suffix, and -mbu- cannot be simply treated as just an allomorph or an alternant of -bu-. The P/C verbs with -mbu- are more lexicalized than the verbs with -bu-. Furthermore, some of them do not have the corresponding active counterparts. Some passive or causative verbs with -mbu- which were listed in the earlier dictionaries have been changed to shapes with -bu- in the standard dictionary YQ. And the later dictionary Yuzhi Zengding Qingwenjian 御製增訂淸文鑑 has introduced new P/C verbs with only -bu-. Some remaining verbs with -mbu- are used to express the extended meanings, and they can be regarded as fossilized words.