When mood states are impaired, daily life is severely disrupted. To maintain a specific mood state, both positive and negative moods must be controlled; however, methods to maintain a positive mood have not been fully established. Previous studies have suggested that heat-killedL. helveticusMCC1848 has the potential to improve positive moods. This study aimed to test the efficacy of heat-killedL. helveticusMCC1848 in maintaining and improving a positive mood with PANAS, a questionnaire specifically designed to assess positive and negative mood, as the primary endpoint. Healthy Japanese nursing students (n= 46) were randomized to receive heat-killedL. helveticusMCC1848 (5 billion/day) or placebo powder for four weeks. Mood state was assessed before and two and four weeks after the intervention began; ingestion of heat-killedL. helveticusMCC1848 significantly improved PANAS ‘Positive Affect’ compared to the placebo. These results indicate that heat-killedL. helveticusMCC1848 is effective in enhancing positive mood.