This study focuses on the performances analysis of a stratified cooling thermal energy storage tank at Universiti Teknologi PETRONAS district cooling plant during the charging operation mode. From the observation, it is noted that problem arises when the chilled water scheduling became inconsistent which led to low performance of the tank due to uneven demands. The objective of the study is to analyze the thermal energy storage tank performance using two methods; thermocline thickness and half cycle figure of merit. Operating data from October 2015 until January 2017 were used for performance assessment. The S-curve temperature profiles were obtained from simulation on temperature distributed in thermal energy storage tank by using the function of sigmoid dose-response and nonlinear regression curve fitting in order to obtain the best fit of the curve. Among the findings; on 11th January 2016 and 11th January 2017, the thermocline thickness results were 5.16m and 7.22â�?m respectively. While for monthly analysis the thermocline thickness for October 2015 was 3.83 m and October 2016 was 5.05m. Based on the results from thermocline thickness and half cycle figure of merit values, the stratified cooling thermal energy storage tank was experiencing deterioration in performance. © 2018 Author(s).