After abolishing martial law in 1987, Taiwan society has gone through not only political liberalization but also social diversification. It has demonstrated the family multi-values of the post-modern society; consequently, relationships between mothers-in-law and daughters-in-law are different from those of the past. Mothers-in-law who own the beliefs of traditional respectful culture face the post-modern daughters-in-law who do not show obedience have to learn how to go through difficulties. Thus, the issues of adjustment mechanisms are worth to be explored. The purpose of this study is to investigate cognition and feeling as the role of mother-in-law in the post-modernism society as well as to understand and analyze mothers-in-law's role changes and adjustment models. Theoretical underpinning is based on Powell's three-level self protection response of adaptation mechanisms. This study utilized exploratory qualitative research, which was conducted with in-depth interviews. There were three major findings from this study. First, in the level of ego defenses, mothers-in-law showed reversion to test daughters-in-law for their filial piety. Second, in the level of coping devices, mothers-in-law used undoing, suppression and sublimation. The third finding is in the level of adjustment mechanisms, in which mothers-in-law demonstrated identification, empathy, anticipation, identification and repression. These findings manifest that understanding and recognizing reality of mothers-in-law in the postmodern society is the important issue for their good adjustment after their losing traditional power. Great expectations lie on giving implications to social workers for their better techniques and skills practiced in the post-modern society.