The present research sought to determine the impact of mentalization-based therapy on improving the mother-child relationship and secure attachment in children. This quasi-experimental study had a pretest-posttest control group design. The statistical population of the study consisted of all mothers of children aged 3-6 in Parand in 2020. Convenience sampling method was utilized and 30 mothers who had obtained lower than average scores on mother-child relationship and attachment scales were randomly selected and assigned to experimental (N=15) and control groups (N=15). The research instruments comprised Mother-Child Relationship Evaluation Scale (Roth, 1980) and Kinship Center Attachment Questionnaire (Kappenberg & Halpern, 2006). The experimental group received training in mentalization-based therapy in 8 two-hour sessions. The collected data were analyzed via MANCOVA. The results showed that mentalization-based therapy was effective in improving total mother-child relationship score as well as those of acceptance, overprotection, overindulgence, and rejection subscales. The results also indicated that mentalization-based therapy was effective in improving the total attachment score and those of positive adjustment/development, negative behavior, negative reactivity, and distancing from caregiver support subscales. Accordingly, it can be concluded that mentalization-based therapy is a practical way to boost the mother-child relationship and secure attachment in children.