The main issues in the informal economy domain are related to its definition, quantification, its position at the level of national economy and the application of econometric models to quantify its effects on economic and social development, at national and regional level. In order to evaluate the informal economy is presented the Ahumadaâ€™s model (2007). Using data series from the official statistics we made estimations of the informal economy for Romania for 2000-2009 time period. The obtained results for the ratio between cash transactions from the informal economy and the ones from the formal economy indicate that this one is situated between 24 and almost 41%. The parameter estimation for the applied regression models was realized using EViews software.