The purpose of this study was to determine the efficacy of Circle of Security (COS) on preschool children & apos; s attachment and well-being. 48 mothers with children & apos; s age 4 to 6 year were randomly selected through cluster sampling and randomly assigned to experimental and control groups (24 mothers and child in each group). Then the experimental group received 20 weekly COS sessions in the center of preschool. The attachment Q-set and Preschool children well-being questionnaire (PCWQ) were used as the pretest, post-test and in the course of follow-up. Results of Multivariate Analysis of Covariance (MANCOVA) showed that the mean scores of attachment, in the experimental group, was significantly greater than the control group in post-test (P = 0.000) and follow-up (P = 0.002). Results also showed that the scores of well-being in experimental group was significantly greater than the control group in post-test (P = 0.006) and follow–up (P = 0.018). The finding of the study emphasized the importance of training of attachment in improvement of preschool children & apos; s attachment and well-being. More studies are suggested to understand better efficacy of maternal behavior on preschool children & apos; s well-being.