With the Thomson scattering (TS) system in KSTAR, temporal evolution of electron temperature (T-e) is estimated using a weighted look-up table method with fast sampling (1 : 25 or 2.5GS/s) digitizers during the 2014 KSTAR campaign. Background noise level is used as a weighting parameter without considering the photon noise due to the absence of information on absolute photon counts detected by the TS system. Estimated electron temperature during a relatively quiescent discharge are scattered, i.e., 15% variation on T-e with respect to its mean value. We find that this 15% variation on T-e cannot be explained solely by the background noise level which leads us to include photon noise effects in our analysis. Using synthetic data, we have estimated the required photon noise level consistent with the observation and determined the dominant noise source in KSTAR TS system.