In-situ measurement was made to evaluate chiller performance and thermal storage density of an ice thermal storage system. The system belonged to a big hotel and the measurement was conducted during late October. Owing to very small cooling load, the data logging was possible for a single thermal storage cycle. However, operation history of the chiller showed a relatively good spectrum of data for performance evaluation. COP and thermal storage density were calculated. The COP at full load was about 4.07, which was lower than $4.8~6.4$ of new chillers. The measured storage density was about $10.9RT-h/m^3;(=152MJ/m^3)$, which also was lower than a criterion of normal performance $(above;13.0RT-h/m^3;or;181MJ/m^3)$. The study result provides technical basis for quantitative ESCO business scenario.