Abstract Introduction ‘Co-sleep’ is defined as the sleep arrangements in which parents and their child sharing a sleeping surface (bed-sharing or room-sharing). Similar to the other Asian countries, Taiwan has a high reported rates of bed-sharing. Previous researches had shown shorter sleep duration and poorer sleep quality in children with co-sleep. However, the association between co-sleep and the children’s emotional and behavioral problems has not been well studied. This study aims to explore the association between sleeping arrangements and children’s sleep, as well as their daytime emotional and behavioral problems. Methods 9,582 caregivers of preschoolers (age= 4.70±0.806; Male: Female=52%:48%) completed a questionnaire regarding their children’s sleep schedule, the Children’s Sleep Habits Questionnaire (CSHQ) and Strength and Difficulties Questions (SDQ). The reported frequency on the items of the CHSQ question regarding co-sleep, asking whether the child falls asleep in parent’s or sibling’ s bed or sleep alone, were used to divide the children into three groups: usually co-sleep group, sometimes co-sleep group and sleep-alone group. Results Among 2,967 preschoolers, 6,272 children (65.5%) reported usually co-sleep, 816 children (8.5%) reported sometimes co-sleep, and 2,494 children (26%) reported sleeping alone. One-way ANOVAs showed significant differences among three groups in: 1) sleep patterns, including weekday nighttime sleep duration (F=24.43, p<.01), weekend nighttime sleep duration (F=3.13, p<.05), weekday nap duration (F=4.24, p<.05), and weekend nap time (F=4.39, p<.05); 2) sleep problems on the CHSQ, including bed time resistance (F=7027.25, p<.01), sleep onset delay (F=33.06, p<.01), sleep duration (F=65.51, p<.01), sleep anxiety (F=788.48, p<.01), night waking (F=37.90, p<.01), parasomnias (F=47.43, p<.01), sleep disorder breathing (F=7.58, p<.01), and sleepiness (F=13.44, p<.01); 3) behavioral problems and development on the SDQ, including hyperactivity (F=21.16, p<.01), emotional symptom (F=23.08, p<.01), conduct problem (F=8.65, p<.01), peer problems (F=20.59, p<.01), and prosocial (F=17.67, p<.01). Conclusion Our results indicate that children with more frequent co-sleep may have shorter sleep duration, more sleep problems as well as more external and internal behavioral problems, while sleep-alone children showed more prosocial behaviors, longer sleep duration, and less sleep problems. The potential developmental problems related to co-sleep may be underestimated in Asian culture and need more attentions. Support (if any):