This study was carried out to investigate the effects of a training program based on choice theory on parenting stress and cognitive flexibility of parents with preschool children. This quasi-experimental research had a pretest-posttest control group design.  The population of the study was comprised of all mothers (120 individuals) who had been referred to municipality-affiliated Tadavom House in Sari in 2019. The research instruments included Cognitive Flexibility Inventory (Dennis & Vander Wal, 2010) and Parenting Stress Index- Short Form (Abidin, 1995). At the screening stage, 30 patients were randomly chosen and assigned to experimental and control groups. The experimental group was administered eight training sessions based on choice theory, whereas the control group received eight training sessions on current social issues. The collected data were analyzed using repeated measures ANOVA (mixed design). The results showed that the intervention based on choice theory increased parents’ cognitive flexibility (effect size for time was 0.653 and effect size for group was 0.331) and reduced parenting stress (effect size for time was 0.803 and effect size for group was 0.416). Accordingly, it can be concluded that the training program based on choice theory is effective in increasing cognitive flexibility and reducing parenting stress in parents with preschoolers. It is suggested that workshops be held at psychological clinics and welfare and education centers using the training protocol based on choice theory.