Background: Obesity, obstructive sleep apnea (OSA), and excessive daytime sleepiness (EDS) are common conditions and are interrelated. Obesity is a risk factor for OSA and independently associated with EDS. We aimed to evaluate frequency of EDS in morbid obese patients with OSA and to identify contribution factor for EDS. Methods: This was a retrospective cross-sectional study in single sleep center. Consecutive patients with OSA (with apnea-hypopnea index 5/h or more) with morbid obesity (body mass index over 35 kg/m2) was enrolled. EDS were defined as Epworth Sleepiness Scale of 10 points or more. Clinical and polysomnographic variables were compared between those with and without EDS. Results: Total 110 morbid obese patients with OSA were enrolled, and 34 (31%) of them had EDS. Those with EDS had higher subjective symptom of insomnia and depression. Rapid eye movement sleep latency was shorter and minimum saturation was lower for those with EDS. Multivariate logistic regression analysis identified insomnia severity (odds ratio, 1.117) and minimum saturation (odds ratio, 0.952) as independent contribution factor for EDS. Conclusions Result of this study suggest that 31.4% of morbid obese patients with OSA have EDS, and it can be affected by insomnia severity and desaturation during sleep.