Objective: In this study, we aimed to analyze the length of stay (LOS) and direct medical costs (DMC) for psychogeriatric inpatients in Taiwan. Methods: We obtained the data from the Psychiatric Inpatient Medical Claim database of Taiwan's National Health Insurance. LOS and DMC of different mental illnesses were analyzed. Results: The average LOS of our study patients was 43.53 days, and the mean DMC was 2,576 US dollars. Dementia was the most common psychiatric diagnosis. Average LOS and DMC were significantly higher in male patients than female (p < 0.001 and p = 0.022, respectively). Over 90% of DMC were non-drug expenses (NDE). The LOS of patients with dementia and major depression was significantly higher for males than for females (p < 0.001). Patients' LOS and DMC showed differences among gender, diagnosis, and type of hospital. The LOS of dementia and delusional disorders and the DMC of major depressive disorders had heterogeneities across hospital types. The results of regression analysis indicated the LOS and the DMC of schizophrenic patients were significantly higher than those of dementia patients (p < 0.001); the LOS of community hospitals was significantly higher (p < 0.001) than that of general hospitals (medical centers and regional hospitals), with the opposite being true for DMC. Compared to public hospitals, the drug expense (DE) was significantly higher in private hospitals (p < 0.04), but LOS and NDE were lower. Conclusion: The determinants affecting differences of LOS and DMC of psychogeriatric inpatients were gender, psychiatric diagnosis, and type of hospital. The DE and NDE of DMC were about 10% and 90%, respectively, but only the NDE showed significant difference based on gender.