Self-compassion is regarded as a mediating or moderating variable in mindfulness-based interventions (MBI). However, few studies have investigated the role of self-compassion on MBI. Therefore, we conducted a randomized controlled trial to examine whether (1) MBI decreases depression and trait anxiety, while increasing trait mindfulness, trait self-compassion, self-compassionate behaviors (SC behaviors), and behaviors, along with improving mood; and (2) SC behaviors moderate the effect of mindful behaviors on mood in daily life. Participants were patients with depression and/or anxiety ( N=19) in Japan. Of the 27 participants recruited, 19 participated in the study. Using stratified randomization, we allocated 10 participants to the intervention group, with an 8-week standard MBI, and nine to the waitlist control group. Depression, trait anxiety, trait mindfulness, and trait self-compassion were assessed using questionnaires, while SC behaviors, mindful behaviors, and mood were measured using an ecological momentary assessment—a method used to repeatedly record events and behaviors in daily life. The results revealed that depression, trait anxiety, trait mindfulness, and trait self-compassion did not significantly change. However, SC behaviors, mindful behaviors, and mood significantly improved with MBI. Moreover, the interaction between SC behaviors and mindful behaviors was significantly shown in the pre-intervention, suggesting that SC behaviors moderate the effect of mindful behaviors on mood in daily life.