This study purported to examine the potential mediating effects of mother-child attachment stability in the relationship between preschooler novelty seeking and harm avoidance temperament and smartphone overuse. One-hundred-and-seventy-three mothers of preschoolers (90 boys and 83 girls) from Seoul and Gyeong-gi provinces participated in the study by filling out a packet of questionnaires measuring preschooler novelty seeking (NS) and harm avoidance (HA) temperament, smartphone overuse, and mother-child attachment stability. A correlation analysis showed significant positive correlations between both NS and HA temperament and child smartphone overuse, a significant negative correlation between attachment stability and smartphone overuse, and significant negative correlations between both NS and HA temperament and attachment stability. Mediational models were tested using regression analyses and the results showed that attachment stability partially mediates the relationships between NS and smartphone overuse and between HA and smartphone overuse. Clinical implications of the research findings were discussed and recommendations for future study were given.