This study attempts to investigate the effect of government expenditure on private investment in Ethiopia over the period 1980-2012. The central question of this study is weather government expenditure has a positive or crowding in effect (complementary hypothesis) or a negative or crowding out effect (the substitutability hypothesis )on private investment in Ethiopia. To achieve its objective it adopted a modified flexible accelerator model to enlighten on the economic relationship between private investment and the other variables and used the modern technique of vector auto regressive model (VAR) and vector error correction model(VECM)as its methodology. The study also used the Johansen-Juselius (1990) cointegration analysis of a multivariate system of equation to estimate the long run relationship between government expenditure and private investment to determine the order of integration of the variable and Granger-Causality test was undertaken to determine causal relationship between the variables. In addition to this the study employs the Augmented Dicky-Fuller (ADF) unit root test and phillip perron test. The statistical tests reveal that all-time series data are non-stationary in their level and they become stationary after diffrencing.i.e.they are integrated of order one I(1).The johansen-juselius cointegration test shows that the series are cointegrated and then employs the vector error correction model moreover the study applies the impulse response function (IRF)and forecast error variance decomposition (FEVD) to investigate the effect of government investment shocks on private investment. And the empirical findings support the complementary hypothesis between government capital expenditure and private investment and that tends to crowd-in private investment in Ethiopia. And the empirical finding of recurrent part of government expenditure shows a mixed effect of complementary hypothesis and substitutability hypothesis which tends to crowd-in and crowd out effect .Thus government expenditure have a positive as well as negative effect on private investment and finally the study is used CHOW test in order to know whether structural break has an effect on private investment or not and the result depict that there is a structural break that have a positive effect on private investment of Ethiopia.