Shyness is significant trait that influences wildly internal and external disorders in adolescents. This study investigated the effectiveness of mindfulness-based cognitive therapy in the treatment of shyness and assertiveness failure puberty girls. The research method was quasi-experimental design with pre-test and post-test with two experimental and control groups. The population of the study is all female high school students (school year 95-94) (7^th and 8^th grades) that studied in Regions 7 and 3 in Tehran. 4 schools were selected using simple random sampling. After a test run shyness and assertiveness in the 4 schools, 30 people who agreed to receive 10 sessions of treatment were selected in available and randomly assigned to two experimental and control group. For experimental groups 10 one-hour sessions of mindfulness-based cognitive therapy was performed. Then both groups were post analysis of covariance was used to analyze the test data. The findings showed that mindfulness-based cognitive therapy have had significant effect to reduce shyness and increase assertiveness (P <0/001). According to the findings of this study, cognitive therapy based on mindfulness as an effective treatment to reduce Shyness and increase the skills of assertiveness.