Objectives: Psychiatric illnesses could cause disability due to their commonly occuring, tendency to recur and chronic character. Thence, psychiatric patients' treatment costs are high. In this study we aimed to evaluate treatment costs invoiced to the social security institution for patients who were hospitalized into Gazi University Hospital Psychiatric Ward in 2015. Method: Quantitative data analysis was executed using software program SPSS 21.0 descriptive values were given such as frequency, percent, mean +/- standard deviation. A Pearson Chisquare Test was used for cathegorical variables, a Mann-Whitney U Test and a Kruskal Wallis Test was used in independent groups lacking normal distribution. Results: There are 389 patients in the study. Schizophrenia (%38.8) was the mostly encountered disease in the patient group, followed by depression (%26.1) and bipolar disorder (%16). Among male patients schizophrenia (%47.6) patients was the most while depression (%31.8) patients was the most among female patients. Average cost per inpatient day was 120.4 TL (44.7 USD), average cost of hospitalization was 3181.0 TL (1182.6 USD). Average cost of hospitalization for men was 2922.0 TL, for women was 3495.3 TL (p = 0.005). Bipolar disorder hospitalization cost was the highest costing 3757.6 TL (1397,0 USD), alcohol and drug use disorder hospitalization cost was the least costing 1864.7 TL (693.2 USD) (p = 0.024). Depression disorders reduce 2.56 times, somatoform disorders 6.25 times reduces the cost of hospitalization than schizophrenia. Average cost per inpatient day in neurocognitive disorders costing 130.7 TL (48.5 USD) was the highest and in eating disorders was the least, costing 107.5 TL (39.9 USD). Conclusion: Payment package applications, as though they provide an ease of calculation, make it harder to discriminate which facility costs for how much. Not every patient is same thus none of their treatment costs could expected to be the same.