El Niño-Southern Oscillation (ENSO) is an important driver of climate variability on interannual time scale in Southeast Asia. Under future climate change, ENSO is expected to be intensified and its two opposite phases as warm El Niño and cool La Niña will be more frequent. Thus, negative impact of ENSO on the functions and services of terrestrial ecosystems is a foreseeable future. In tropical ecosystem, the secondary forestlands are increasing because of implementation of reforestation policy in many tropical countries. Therefore, the secondary forests are likely to have more roles on regional and global ecosystem function and services. In the present study, we investigated the response of the secondary dry dipterocarp forest phenology to El Niño events during 2009-2016. The forest phenology was evaluated by extracting the Normalized Difference Vegetation Index (NDVI) from Moderate Resolution Imaging Spectroradiometer (MODIS). The micro-meteorological data were collected at the study site in Ratchaburi Province that included rainfall, temperature and soil moisture. We found that El Niño in 2009/2010 and 2015/2016 led to a dramatic decline in NDVI. The impacts of these El Niño on forest phenology were characterized by the late start of growing season and shorter in the length of growing season when compared to other years