One of the characteristics of the venture capitalist's investment is sequential injection of capital. This practice is adopted in order to reduce agency problems and to give flexibility to the investment done by the venture capitalist (VC). Thus, at each stage, the investor can decide to stop his investment if the firm's flows don't evolve in the right sense. The aim of this paper is to develop a two stages theoretical model which permits to settle the milestones which release the VC's investment, by considering that the flows generated by the firm are uncertain and that they follow an arithmetic Brownian motion. The model considered here is based on a real options approach and takes into account i) the VC's risk aversion and ii) the proportion of capital allocated to the VC in return of each investment.