The national fiscal budget is always the government single most important activity that provides answer(s) to the basic political economic questions: who gets what?, How or to what extent?, when and where? Therefore, the roles of the legislature at ensuring that effective budget and reliable budgeting process is undertaken to address these political economic problems is inadequate. It is against this significant role played by budget that this study seeks to consider the inadequacy as well as oversight roles of legislature for the realization and applaud of effective budgeting process under the presidential system in Nigeria. To this end, secondary data has been sourced for the study. The study used content analysis. The study revealed that several challenges confront the legislature in Nigeria. These include inability to access independent information on budget performance; dominant political party influence; inadequate time as well as inexperience on the part of the legislature as a result of prolonged military rule in Nigeria. The study therefore recommends that for the legislature to perform its roles effectively, free access to information is essential, reduction in the roles of political parties and adequate training, as proper training of the legislature in a sustained democratic rule is crucial. The study seeks to consider the inadequacy and the oversight functions of the legislature in order to ensure the realization of effective budgeting process under the presidential system in Nigeria.