This paper empirically examined the short-run and long-run dynamics among external public debt and foreign exchange reserve of Ethiopia. The two variables are playing a pivotal role in the growth and development of nations economy. To achieve the objective the study took 39 years data from the year 1981 to 2019 from National bank of Ethiopia and World Bank data sets. The study used descriptive analysis and empirical methods of analysis. The Autoregressive Distributed Lag model with error correction models were employed after checking the possible assumptions of our economic series. The results of ADF test statistics confirms our economic series are stationary at level and first difference forms. Bounds co-integration test suggests one co-integrating relationship between the variables taking foreign exchange reserve as the outcome variable. According to the descriptive method of analysis, on average, in Ethiopia the trend for service sector indicated that an ever improvement of the sector throughout the periods and supplementing the notion of change from agriculture base to service sector. In addition, the trade tariff rate of Ethiopian economy is indicating a downward movement and this in turn justifies the relative openness of the economy to the globe. In the same manner the financial development indicator of the nation is rising, which assures relative improvement in the financial sector. On the other hand, according to the Autoregressive Distributed Lag model in the short -run average trade tariff rate, share of manufacturing sector from the GDP, and lagged value of EPD itself predicts the external public debt significant at least at less than 10 percent level of significance. Moreover, the error correction model revealed that in the long-run, financial development indicator, debt service payment, and average trade tariff rate were predicting the stock of foreign exchange reserve for Ethiopian economy. The result also indicates that in the short-run, only the share of agriculture and service sectors are significantly predicting the variations of the stock of foreign exchange reserve, ceteris paribus. Finally, the concerned body specially the government of Ethiopia should limit or reduce the amount of external debt inflows that has an adverse effect on debt service payment, and recheck the budget sources for financing different projects especially manufacturing industries rather than highly basing on external sources in the form of external public debt . More importantly, the government should enhance the value of export potential, among others.