||This paper deals with an analysis of tax system called the flat tax which was created in theoretical works of economists Robert Hall and Alvin Rabushka. First, in order to illustrate why a debate on tax system change is appropriate even from an economic point of view, we analyze the existing federal income tax in the US. Then we present the flat tax proposal and its positive impacts on economy, which are expected to come as a consequence of tax simplification and lower marginal rates. Again, from an economic point of view, some of these allegedly positive impacts don’t seem to be positive at all. Therefore, we question some of the targets and presumptions of the flat tax. The most important critique is a thesis that increases in tax system efficiency lead to increases in government size and hence to higher degree of rent seeking. In the following section we analyze the concept of rent seeking itself and by establishing a connection between rent seeking and legislation, we link it to the debate on flat tax. Then we use empirical data to show the effects of rent seeking and rent avoidance on legislation. Finally, we conclude that it is impossible to find clear evidence of any of the assumed flat-tax effects, because there are many other hardly identifiable factors aside from the tax reform that can have impact on economic performance.