The present study was undertaken to examine the effectiveness of positive thinking skills training on high school girls’ achievement motivation in Tehran. The study was a quasi-experimental one with pretest/posttest/follow-up design. The population of the study comprised all girls studying in Tehran’s private high schools (district five). For the purpose of sampling, one high school was selected through convenience sampling method and in the next step two classes were randomly chosen. The instrumentation included Hemans Achievement Motivation Questionnaire. At first, a pretest was administered to both experimental and control groups and next the control group received eight 90-minute sessions of positive thinking skills training, whereas the control group did not receive any intervention. After completion of the training, both groups were given the achievement motivation test. The data were analyzed using ANCOVA and the results showed that the experimental group’s mean scores on the achievement motivation test were 82 and 93 in the pretest and posttest, respectively. Furthermore, the inferential analyses confirmed the effectiveness of the positive intervention on students’ achievement motivation with 99% confidence interval. Hence, it can be concluded that positive thinking skills training significantly increases students’ achievement motivation (p>0.05). As a result, it seems necessary that positive thinking skills be taught to high school students.