The aim of this research was to study an analyze the relationship between the seasonal rainfall over Thailand and ENSO in area of Niño3 and Niño3.4. The rainfall data are from APHRODITE’s and Thai meteorological department. The rainfall variability over Thailand was analyzed by using empirical orthogonal function (EOF) technic and the relationship between principal component time series (PC’s) of rainfall with Niño3 and Niño3.4 index was calculating by Pearson’s correlation method. The results shown that the Niño has the most correlation with rainfall during March April and May. The Niño3 and Niño 3.4 SST index leads the standardized PC values in zero lag time. The composite analysis was used to divided stong ENSO event. These result demonstrate that ENSO event (phenomenon) can be considered an important factor in rainfall behavior over Thailand.