Health care for older people is featured by simultaneous management of the multiple comorbid complex conditions, including acute illnesses, underlying comorbid conditions, nutritional status, functional status and many others. The purpose of this study was to investigate nutritional status and its effect on the relationship between co-existing diseases and functional status among older patients admitted to the geriatric ward in Taiwan. A descriptive correlational design was used and data of 401 patients were retrospectively collected for analysis. Comprehensive geriatric assessment was performed for all patients, which included the Mini-Nutritional Assessment, the Barthel Index, and the Charlson Comorbidity Index. The Tobit model was used to manage censoring data, and hierarchical logistic regressions were conducted to determine the relationships among nutrition status, functional status and comorbid conditions. A significant mediating effect of nutritional status on the relationship between multimorbidity and functional status was found. Although multimorbidity, nutritional status, and functional status are usually interlinked in geriatric care, results of this study showed that nutritional intervention may play a stronger role in improving functional status of older patients admitted for acute geriatric services. Further intervention study is needed to confirm the mediating role of nutritional status between comorbidity and functional status for older patients.