Vegetation phenology is a key environmental indicator of ecological response to climate variability. Long-term studies of phenological changes by applying remote sensing techniques can improve our understanding of spatiotemporal climate patterns. However, studies have not been focused on such phenological dynamics in Tana and Beles sub basins. Therefore, this study explored the vegetation phenology dynamics in relation to rainfall seasonality parameters and lag time effects of rainfall on vegetation phenology. From MODIS 8-day normalized difference vegetation index and from Climate Hazards Group Infrared Precipitation with Station dataset pentad rainfall data was calculated over the 20-year period (2001-2020). TIMESAT program was used to generate and derive major phenological events for cropland, grassland and shrub and bushland vegetation types. Rainfall seasonality and phenological event relationships were examined, and simple linear regression models were created. The study found cropland, grassland, and shrub and bushland had mean start of growing season on 166th, 133rd and 146th days of the year, with average end of growing season dates 312th, 335th and 343rd days of the year, respectively. Over the 20-year study period, rainfall and phenological parameters exhibited a positive and significant strong relationship with correlation coefficients of 0.76, 0.49 and 0.56 for the start of the growing season and the start of the rainy season, and 0.31, 0.38 and 0.19 for the end of the growing season and the end of the rainy season respectively. Cropland, grassland and shrub and bushlands had average 23,11, 6 days lag time between the start of the rainy season and the start of the season, respectively. As a result, the findings of this study point to a much more explicit role of rainfall on vegetation phenology, specifically as one of other climate agents that could recommended as properly understand the pattern of phenological events which helps farmers and decision makers about the determining crop sowing date and vegetation management that able to adapt the changing environment.