In developing countries, such as, Ethiopia, rainfall is the crucial factor for determining Gross domestic product (GDP). In this study, different types of statistical analysis were performed to understand the reasons behind the rainfall variability from monthly to inter decadal levels during 1981 to 2017. With the exception to rainfall from December to March, although not statistically significant, increasing trends have been noticed during most of months. During the El Niño, a greater reduction in the summer season's rainfall Kiremt (June to September) noticed, whereas, enhancement during the end of winter to spring season Belg (February to May) observed. In contrast, during La Nina an increasing trend observed for the Kiremt, but decreasing trend observed for both Belg and Bega (October to January). Most of these changes indicate the role of Tropical Pacific Ocean's sea surface temperature (SST) coupled with atmospheric circulation regulating principally the rainfall of the study area. The Kiremt rainfall shows statistically significant correlations with a time lag of about one month with Nino 3.4 and Nino 3 standardized SST anomalies(r = - 0.36 and r = - 0.37 respectively at 95% confidence level).The monthly and seasonal rainfall cumulated to the annual increase of 4.12 mm/yr at 95% confidence level, with increasing (decreasing) trend during La Nina (El Niño).The decadal trends in rainfall from 1981 to 1990 and from 2001 to 2010 are consistent with the above observations, however, with the exception in between decade1991 to 2000, and 2011 till 2017.