It is a widely held opinion that the attainment of a robust economic growth and development in Nigeria will require, among other things, an increase in investment which will have to come primarily from the private sector. Viewed against the background of growing evidence of a link between investment and economic growth, the question of what determines private investment behavior in Nigeria therefore becomes an important one. Several studies in developing countries emphasize the importance of macroeconomic policy in explaining variations in investment, and in particular, identify the macroeconomic determinants of private investment to include; interest rates, output growth, public investment, bank credit to the private sector, inflation, real exchange rate , and the level of trade. This study proceeds in the same vein and evaluates the macroeconomic determinants of private investment in Nigeria by means of an estimation technique based on the co-integration and Vector Error Correction Model (VECM).