Background Day care service (DCS) provides various activities in a professional environment to meet the old people with functional limitations. However, relatively little is known about the effects of DCS on physical and mental functions. Methods This was a retrospective study that we used a comprehensive geriatric assessment to evaluate the changes before and after DCS among participants in a hospital-affiliated geriatric day care center in Taiwan. The burden of the participants' families was also assessed. Results The 18 participants with a median age of 80.9 (interquartile range (IQR) 75.2-86.6 y/o) were enrolled and followed up for 6 months. Based on the clinical dementia rating (CDR), disease stage was very mild in 3 participants, mild in 10, moderate in 3, and severe in 2. The activities of daily living (ADL) scores of the participants improved significantly from 75 (IQR 60.0-80.0) at baseline to 77.5 (IQR 65.0-90.0) at the 6 month (p < 0.001), and mini-mental state examination (MMSE) scores from 15 (IQR 11.5-20.0) to 18 (IQR 15.8-24.0) (p = 0.026). There was a positive correlation of baseline mini-nutritional assessment-short form score and the 3-level version of the European Quality of Life-5 dimensions utility index with both ADL and MMSE scores at the 6-month follow-up. In addition, the family burden scale was reduced from 22 to 15 (p = 0.002). Conclusions The physical and cognitive functions in old people with dementia who received DCS were maintained or partially improved, and their families' stress burden was alleviated.