The relationship between population growth and economic development is controversial. The purpose of this paper is to test the link between population growth measured by the fertility rate and the economic development measured by the GDP per capita in Japan as a developed country and in Ethiopia as a non-developed one. The autoregressive distributed lags model ARDL is applied on Ethiopia and Japan for the period 1991-2022. Causality between variables is tested using granger causality test and the coefficient of correlation is calculated. The findings are the existence of a significant inverse causal relation between the population growth and the economic development in Ethiopia in the short run and the long run. However, there is no significant causal relation in Japan. The study conducted tests to assess the reliability of the model such as serial correlation, heteroskedasticity, normality, and stability tests. The results indicated that the model was stable. Furthermore, there were no indications of serial correlation or heteroskedasticity