This study examines the determinants of investment decision of quoted manufacturing firms’ in Nigeria. The study is motivated by the fact that establishing the link between cash flow, firm size and capital structure will provide an insight into the activity of manufacturing firms in Nigeria. The study collected data from thirteen (13) manufacturing firms’ annual financial statements. The data collected were analyzed using ordinary least square regression. In all the sub sector analysis, it was found that cash flow and firm size are the major determinants of investment decision by manufacturing firms in Nigeria. It is therefore recommended that while cash flow can affect the investment decision of firms, it is also important that firms in this sector take into consideration the type of capital structure they have before embarking on investment. A firm that is largely financed by debt capital will need to borrow more to carry out further investments which could consequently result in total control by outsiders.